Attempting to add a BCC recipient has recently begin failing in every instance: whether to self or to a second recipient... the original email goes through correctly.
Using Win7, PMv4.73
An example:

220 smtp.mail.yahoo.com ESMTP ready
<< EHLO [nnn.nn.nnn.nn]
250-hermes--production-ne1-7864dcfd54-q5j27 Hello [nnn.nnn.n.nnn] [nnn.nnn.nnn.nn])
250-PIPELINING
250-ENHANCEDSTATUSCODES
250-8BITMIME
250-SIZE 41697280
250 AUTH PLAIN LOGIN XOAUTH2 OAUTHBEARER
<< AUTH LOGIN
334 xxx
<< xxxxxxxxxxxx=
334 xxxxxx
<< xxxxxxxxxxxxxxxxxxxxxx==
235 2.7.0 Authentication successful
<< MAIL FROM:SENDER@DOMAIN.com SIZE=1450
250 2.1.0 Sender SENDER@DOMAIN.com OK
<< RCPT TO:<BCC RECIPIENT@DOMAIN.net>
250 2.1.5 Recipient <BCC RECIPIENT@DOMAIN.net> OK
<< DATA
354 Ok Send data ending with <CRLF>.<CRLF>

554 6.6.0 Error sending message for delivery.

=====
Thos

I just tested sending three messages containing two BCC addresses. Each one was sent using a different SMTP service (GMX, Zoho, and Outlook). No problems with any of them.
I don't have any idea what might be causing your problem.

FWIW, my PMail is configured to suppress BCC recipients but I don't think that would have any impact on your problem.

Where are you entering the BCC email address??

Should click on the Special tab, and put the addresses in the Blind CC:
looks like you are putting BCC in the regular To: listing, so you have BCC space email address, which would be invalid with a space in address.

Not sure how you edited info.

Exactly how are you enter BCC addresses.

Yes! I have not been able to use Pegasus's BCC facility for many months now and it is so aggravating. Today I have tested thoroughly. Adding a CC in Pegasus results in the CC person's address receiving the CC copy every time! No problem. But when any email address, be it a Yahoo suffix, a Gmail suffix, or whatever, is placed in the "Blind CC" box under the "Special" tab in Pegasus, two error messages are always received. The first is in a Pegasus pop-up window, which refers to "pipelining". The second is in the returned message, which states:

Delivery has failed on the enclosed message for the following
reasons reported either by the mail delivery system on the mail
relay host or by the local TCP/IP transport module:
554 6.6.0 Error sending message for delivery.

Part of my testing included sending BCC copies from my Yahoo email app, my Gmail email app, and my K-9 email app. From each of those BCC copies wing away to their recipients every time without any problem.

Please note, I have changed none of my settings in Pegasus. However, perhaps it is now necessary to do so, though I can't think of any settings affecting the BCC facility. Can anyone solve this situation?

Couple things.

1. 554 error seems to come from a mail server saying your are sending too many emails at same time?? How many people is the email going to??How many in TO: FieldHow many in CC: FieldHow many in BCC: FieldWhat is total of all the above.

Don't know if there is one limit for all, or if they might have different numbers for each group.

I was just able to send BCC message thru my local ISP to gmail acccount and from gmail account to local isp, and both had no errors.

1. Exactly what version of Pegasus are you using. Assume your created log file. to see results.

Recall some servers had limits of how many users could be included in a single message. Recall there was an option to have it send individual messages to each user, rather than sending a single message that gets sent to others. That makes sense, but some server then have limit.

Might need to split the sending to smaller groups.

Would be intereting to see what actual problem is...

https://senders.yahooinc.com/smtp-error-codes/ has some info from yahoo about code.

Note clear if it is saying that it might be just one of the email address cause problem, or if it will then block all the address.

But seems to point to server issues or bad emails more the client being used.

Are addresses being individual entered or copy paste or from a distribution list.

Thanks for the thoughts above. Answers are:

@idw Yes, K-9, Yahoo, and Gmail all function well now despite OAuth2, but shouldn't Pegasus do so too, now that I've used the Google password generator and no longer have any trouble retrieving Gmail emails into Pegasus?

@msetzerii Yes, perhaps the server is the issue.

When I successfully send BCCs from the Gmail app on my Android devices, I imagine they go via smtp.tools.sky.com, as that is the smtp server address which I have entered under Server Settings > Outgoing Settings.

When I successfully send BCCs from the Yahoo app on my Android devices, I imagine that email goes via the Yahoo server. (One can't enter a server in Yahoo for fetching or sending, and the server is not displayed anywhere in the settings.)

When I successfully send BCCs from the K-9 app on my Android devices, that email goes via smtp.tools.sky.com.

And when I unsuccessfully send BCCs from Pegasus Mail on my computer, that email goes smtp.tools.sky.com.

People with greater knowledge of the internet and its workings than myself may be able to tell me why BCCs can go via smtp.tools.sky.com from the Gmail and K-9 apps on my Android devices, but not from Pegasus Mail on my computer.

Re. the number of mails being sent, in my testing of all of the above I have sent to only one email address in the "To" box and to only one email address in the "BCC" box.

Re. the version of Pegasus I am using, it's the latest: version 4.8.

Thanks for the Yahoo link offering advice. With regard to the 554 SMTP error which I and @Joshua Ramsepm have received, that link says that 554 indicates one or more of the following. (P.S. Note the red font in the following lines is not intended. It appears to be due to a glitch in the "Reply" function of this website. Ignore the red font.)

You're trying to send a message to an invalid email address.  
(But no, I am not doing that.)

Your message failed authentication checks against your sending domain's DMARC or DKIM policy. 
(Perhaps.  So what can we do?)

The message contains characteristics that Yahoo won't accept for policy reasons.
(The test messages contain just a few simple words, so this is not the reason.)

Other suspicious behavior which leads Yahoo to issue a permanent rejection for your SMTP connection.
(Perhaps.  In which case it is Pegasus Mail which is acting "suspiciously" and we need to work out what that "suspicious behaviour" is.)

Your IP is listed by Spamhaus. Please check with https://www.spamhaus.org.
(Perhaps.  However, other people on other devices in my home have the same IP address and they have no problem sending BCCs.  But I will try to contact my server, Yahoo-Sky, tomorrow, and ask them if they can shed light on the problem.)

There is interesting information at https://senders.yahooinc.com/best-practices/
(Up until a year ago I used to send out emails via Pegasus Mail's Distribution List facility to a group of about 50 people, and it's possible that someone who no longer wanted to be on my Distribution List did not ask me to remove them but instead reported one of my Distribution List emails as Spam.)

@idw @Joshua Ramspm As my careful testing has proven that

the server "smtp.tools.sky.com" allows BBCs from the Android apps Yahoo, Gmail, and K-9,

but that the server "smtp.tools.sky.com" does not allow BBCs from Pegasus,

then surely there is something in Pegasus's creation of, or framing of, a BCC which causes "smtp.tools.sky.com" to refuse to pass on Pegasus's BCC - and thus to advise, via error message, that "pipelining" is occurring (and that pipelining is not allowed)?

Question: in your Pegasus program, when you "Start a new message", if you go to the "Special" tab, what settings do you have there? In my Pegasus program, only "Use Mime features" is ticked.

Couple of things.
Under tools internet option sending edit
what the options set or checked.
under General

under Security.

Do you have login to SMTP server checked and have the userid and password set.

Question: Does it allowing you to send regular To: or CC: emails to the accounts or do they also fail?

Do you have an email address on this server that we could send a BCC message too and see what our log files shows.

Do you have a copy of items that get thru with other programs and see what header lines they might have that the Pegasus one doesn't??

Just in case it helps anyone here to solve problems in sending BCCs from Pegasus Mail, here is the text of the Pegasus Mail pop-up which appears when my sending of a BCC fails.

SMTP Network or Protocol Error sending mail.
An error has occurred during an SMTP mail transaction. The details of the error are shown below, and if one is available, a trace showing the communication between Pegasus Mail and the server appears at the foot of this window. If a trace is present, you can copy text from it by select it, right clicking, and choosing "Copy".
Hostname: smtp.tools.sky.com
Name of SMTP profile: smtp.tools.sky.com
Description of error: Protocol error: the server has unexpectedly refused or failed a request made by Pegasus Mail.
Trace showing end of transaction: [note that I have removed sensitive info from the following trace].
17:57:49.481: >> 220 smtp.mail.yahoo.com ESMTP ready
17:57:49.481: << EHLO [IP number removed]
17:57:49.501: >> 250-hermes--canary-production-ir2-[code removed] Hello [IP number removed] [IP number removed])
17:57:49.501: >> 250-PIPELINING
17:57:49.501: >> 250-ENHANCEDSTATUSCODES
17:57:49.501: >> 250-8BITMIME
17:57:49.501: >> 250-SIZE 41697280
17:57:49.501: >> 250 AUTH PLAIN LOGIN XOAUTH2 OAUTHBEARER
17:57:49.511: << AUTH LOGIN
17:57:49.541: >> 334 [code removed]
17:57:49.541: << [code removed]
17:57:49.561: >> 334 [code removed]
17:57:49.561: << [code removed]==
17:57:49.741: >> 235 2.7.0 Authentication successful
17:57:49.741: << MAIL FROM:<email address removed> SIZE=1285
17:57:49.881: >> 250 2.1.0 Sender <email address removed> OK
17:57:49.881: << RCPT TO:<email address removed>
17:57:49.901: >> 250 2.1.5 Recipient <email address removed> OK
17:57:49.901: << DATA
17:57:49.931: >> 354 Ok Send data ending with <CRLF>.<CRLF>
17:57:50.501: >> 554 6.6.0 Error sending message for delivery.

And as stated earlier in this thread, the second error message is received when my server returns the BCC email to me as "Undeliverable" along with the following message:

Delivery has failed on the enclosed message for the following reasons reported either by the mail delivery system on the mail relay host or by the local TCP/IP transport module:
554 6.6.0 Error sending message for delivery.

Question: Is the returned email for just one of the BCC addresses, some of the BCC addresses or All of the BCC addresses.

I'm in Guam, so our IP block falls in the Asian region, and I've run into some sites that block us since we don't have a continental US IP. ISP has a proxy server with a US IP, and using it usually works. Have some email lists that won't take my standard ISP email address, so have to use gmail one for them.

554 error doesn't seem to say exactly what is issue?

From https://bobcares.com/blog/554-permanent-problems-with-the-remote-server/

554 error code means that the receiving server sees something in the From or To Headers of the message, that it doesn’t like.

So, it could be anything that there setup is looking for. Something it doesn't like, or something it wants to see that isn't there.

Well, perhaps someone will figure out what it is.
Good Luck.

In addition to the BCC, was there also a "To: " address in the data?

Yahoo have been known to reject emails with 6.6.0 code if there is "BCC:" but no "To:" address as a supposed anti-spam measure. The fact that the message was rejected after the DATA section was accepted suggests that Yahoo may well have parsed the data and rejected the email on that basis.

You should ensure that there is a valid To: address in an email, even if it is just the same as the "From:" address.

@Stuart McLachlan, yes, all of the messages which have failed in the above way have been sent to an email address, i.e. in the "To:" box. I'm currently thinking the answer lies in the word "pipelining" in the trace which Pegasus shows after the failure. I wonder if "pipelining" means one can't send a BCC to another user who is either using the same server or who is linked to one's own email account. I experience the failure message most often when I BCC my partner who of course uses the same provider, and uses one of the other addresses registered to our account. What do you think?

Pipelining is not the issue. Getting a PIPELINING response to EHLO just means that the mail server supports it. WIth pipelining, you can send a series of commands without waiting for individual responses and the server will process the batch of commands. From the trace, it is apparent that Pegasus Mail is not using pipelining (it is waiting for a response after each command)

Do you have "Suppress BCC field llistings in outgoing mail" checked?
If so, the BCC recipients address will be replaced by the text (Suppressed) and the recipieint address will not appear anywhere in the DATA block.

IN that case, it is quite possible that the mail server is rejecting the message to the BCC recipient under a spam filter rule

Read the Pegasus Mail Help for that setting. If it is set, try un-setting it.

@Stuart McLachlan, my mail server must have recently begun rejecting BCC addresses which are "suppressed" owing to a revised spam policy.

Yes, by going into Pegasus's Options > General Settings > Outgoing mail > Sending mail
and by then DE-SELECTING "Suppress BCC listings in outgoing mail"
I can now send BCC copies to any email address I add.

Having changed the above setting, I have tested whether the identities of BCC recipients can be seen by whoever receives the email, by examining the received email using Pegasus's "Raw Data" view. And unfortunately, because I had instructed Pegasus to no longer suppress the BCC addresses, they are visible in the Raw Data view.

I have asked a recipient who does not use a sophisticated program like Pegasus, if they can now see the identities of BCC recipients and they have said they can't.

So one is going to have to be very careful in future, always keeping in mind that any recipient who is determined enough to find out if anybody else has received a copy of an email can now do so. This makes using Pegasus's BCC unreliable, and so one may just have to get into the tedious habit of saving a copy of an email to one's own folders and then opening it and forwarding it to people to whom one used to be able to reliably send a BCC copy.

I suppose that the above finding means that when I have been confidently sending off BCCs using various apps on my Android devices, the main recipients of those emails could, in theory, have been examining their raw data to see if BCCs were sent to anyone.

Have you considered using distribution lists instead of BCC? They can be configured to suppress the list of recipients and are pretty easy to modify on the fly. You could create one and modify its list of recipients per use or if create a static on if your recipient list is always the same. You would need to test whether the dlist suppression mechanism triggers the same error..