Most, but not all, Gmail addresses that I send to are now getting bounced with the following message appearing instantly in my new mail box:
Mail delivery failed: returning message to sender
Seems as though its coming from Pegasus not from a server.
I’ve looked on line for solutions but everything I find seems to be relate to adding a Gmail account to Pegasus, and that is not what I want to do.
If I send to the same addresses using a Gmail account I have there is no problem, but I only use this account occasionally for non-business purposes so that isn’t a solution either.
I have whitelisted the email addresses.
I updated to PMail ver 4.8 this morning and noticed something in the release notes about Gmail protocols but again, this does not relate to the bouncing emails so far as I can see. Nor has it helped.
I’ve been a PM user for decades and would not be without it, but this has got me stuck and any help would be very much appreciated. I’ve tried searching the forum but can’t see anything that is likely to hit the spot.

Enable internet session logging and then send a message to one of the failing address. The resulting smtp log should provide a clue into the point of failure.

Enable internet session logging in the General tab of Internet options.

Log files are written to a directory named TCPLogs which is located in your home mailbox directory. Log file are plain text files.

Do not post the entire content of a log file. It contains easily decrypted authentication information. You can remove that information by deleting the four line following the "<< AUTH LOGIN<cr><lf>" entry. The fifth line should indicate whether or not authentication was successful. That line will be important to include.

Very many thanks Brian. Typically this hit me at a busy time, so sorry for the delay. The lines you asked for are:
06:04.490: >> 250-AUTH LOGIN PLAIN<cr><lf>
(four other lines)
10:06:04.493: >> 250 OK<cr><lf>
I notice this morning that the fourth address, which was working, now has the same problem.

Look at the bottom of the log file for a line that says:
<< QUIT<cr><lf>

Just above it will the information about whether the server accepted the message for delivery. Check that out.

Also, doesn't the mail deliver failure message contain an explanation?

The final lines in the smpt file after the end of the message text are:

10:06:04.606: << <cr><lf>
10:06:04.607: << <cr><lf>
10:06:04.608: << <cr><lf>
10:06:04.609: << <cr><lf>
10:06:04.609: << <cr><lf>
10:06:04.610: << <cr><lf>
10:06:04.611: << .<cr><lf>
10:06:04.612: >> 250 Mail queued for delivery.<cr><lf>
10:06:04.616: << QUIT<cr><lf>
10:06:04.651: >> 221 Closing connection. Good bye.<cr><lf>
10:06:04.652: --- Connection closed at 20 Jun 2023, 10:06:04.652. ---
10:06:04.652:

The mail delivery failure message only says:

This is an automatically generated Delivery Status Notification.
Delivery to the following recipients failed permanently:

• n**2@gmail.comReason: A message that you sent to the following recipient could not bedelivered due to a permanent error. The remote server ?? respondedwith: n**2@gmail.com ??:?? This message was created automaticallyby mail delivery software on the server ..

The from field in the header only had <> in it

This entry in the log confirms that the message was accepted by the destination server. Pegasus Mail did its job. The destination server rejected if for some reason (blacklisted, spam detection, potentially hazardous content). I lean towards the IP address of your sending server being on a blacklist. That IP address will be in the log. You can check it on https://mxtoolbox.com/.

Very many thanks Brian, that removes a variable. So I'll get to grips with MXtoolbox in the morning, but at a glance it looks like being quite a steep learning curve.

Brian: I realise that this has now gone beyond the bounds of PMail, but in case you are still watching, mxtoolbox.com gives me a screen with:

Cross - DNS Record published - DNS record not found
Tick - DMARC Record published - DMARC record not found
Tick - DMARC Record not enabled - DMARC Quarentine/reject policy enabled

Beyond that they want a rather large (refundable) subscription, which I am reluctant to do unless it becomes unavoidable. I assume that the next place to go is my ISP and I am now doing that. I will post the outcome in due course as I suppose it could help someone else.

Just in case it is of any help to someone else with the same problem, I raised this problem on my ISP's community forum. As I use a domain hosted elsewhere, it was suggested that i add this test to the domain settings;

v=spf1 a mx include:_spf-internal.plus.net include:_spf-internal2.plus.net ~all

That has fixed the problem. Many thanks again for establishing that I needed to look elsewhere for a solution.