My messages to other peoples' gmail.com addresses have started being bounced, and the fix I need is to set up my SMTP server use properly. I've got it to work in another (lesser) email client, but I've struggled with configuring Pegasus Mail.

It's not my SMTP server. The settings specified by the email provider are
Security - SSL
Outgoing Port - 465 or 587
Authentication - Password

I've tried various combinations of direct SSL or STARTTLS, port 465 or port 587. Haven't found one which works yet.

Here's the trace showing the SMTP transaction for direct SSL, port 465. What do I need to adjust?

10:07:28.687: [*] Setting SNI hostname to 'mail.secure.aluminati.net'
10:07:28.750: [*] SSL/TLS session established
10:07:28.750: [*] ECDHE-RSA-AES128-GCM-SHA256, TLSv1.2, Kx=ECDH, Au=RSA, Enc=AESGCM(128), Mac=AEAD
10:07:28.750: [*] Peer's certificate name is '/CN=mail.secure.aluminati.net'.
10:07:28.772: >> 220 svc01-2.prd.rdg.aluminati.org ESMTP Postfix
10:07:28.772: << EHLO [192.168.1.148]
10:07:28.772: >> 250-svc01-2.prd.rdg.aluminati.org
10:07:28.772: >> 250-PIPELINING
10:07:28.772: >> 250-SIZE 31240000
10:07:28.772: >> 250-VRFY
10:07:28.772: >> 250-ETRN
10:07:28.772: >> 250-AUTH PLAIN LOGIN
10:07:28.772: >> 250-ENHANCEDSTATUSCODES
10:07:28.772: >> 250-8BITMIME
10:07:28.772: >> 250 DSN
10:07:28.787: << AUTH LOGIN
10:07:28.803: >> 334 VXNlcm5hbWU6
10:07:28.803: << <REDACTED BY ME>
10:07:28.818: >> 334 UGFzc3dvcmQ6
10:07:28.818: << <REDACTED BY ME>
10:07:29.934: >> 535 5.7.8 Error: authentication failed: UGFzc3dvcmQ6

Ugh. I think I've got it. The service I've got hasn't enabled SMTP access for my account (just a forwarding address). Not at all Pegasus Mail's fault, but a clearer error report for "SMTP username and password combination rejected" would have been helpful.

Pity, because I've been using that address since 2006, and don't want to have to change it.

You say that you have only had problems with mail delivery to gmail addresses but then say that you provider has not enabled SMTP. This is confusing since SMTP had to be enabled for delivery to other addresses. The failure to authenticate adds to that confusion. Were you able to send to other email addresses prior to attempting to change SMTP host settings?

Know that Gmail announced that it was implementing stricter security requirements effective in February. This is only supposed to apply to bulk mail servers. If the gmail delivery problem just started, perhaps aluminati.net is considered such by Google and does not meet their new security requirements.

"10:07:29.934: >> 535 5.7.8 Error: authentication failed: UGFzc3dvcmQ6" appears to indicate an error in the supplied credentials (either typo or non-existent).

You mentioned that SMTP access is not enabled for your account. I presume you mean that you found that you don't have a valid set of credentials to use the SMTP relay of your mail service provider? That scenario would also result in the error message in your original post above.

If that is not the case, then the only thing i can suggest is to double-check the credentials supplied and the login format (as i recall at one point in the past i had an email provider who did an unannounced backend change and their new server software didn't accept the email+domain.com login format that i had been using up to that point, and i had to change the login to email@domain.com format).

Brian and limpeh, thank you for the replies. Here's a slightly longer version of what appears to have happened, with some of my initial confusion omitted. TL;DR it isn't a Pegasus Mail issue.

I use, for almost all personal email, an address provided "free for life" my my university. It was originally just a forwarding alias, but has, without my really noticing it, now become a free subscription to a webmail service with the address set up as the same forwarding alias. With names changed, that means that messages sent to memyselfi@university.net are being delivered to memyselfi@outlook.com. I have been sending messages with the alias address configured as the From: field, using my ISP's or Microsoft's (Outlook) SMTP service for nearly 20 years without significant problems. The SMTP and final address have had to be switched several times, but my "for life" email address has been stable.

In the last few weeks, though, messages to some gmail.com addresses have been bounced with the response

smtp; 550-5.7.26 This mail has been blocked because the sender is unauthenticated.

This started happening just when I needed to send messages promptly and without messing about. Internet rumours eventually suggest the issue is that the alias email address doesn't have all the necessary anti-spam references to the SMTP server I was using. When I got that far, I thought, well, send emails as memyselfi@university.net by using that address's matching SMTP server. I looked up the server specification (mail.secure.aluminati.net) and tried to enter it into Pegasus Mail so as to be able to send messages, and had problems.

I was mistaken in thinking this had worked with some other service. It turns out that my free for life service includes IMAP (which I'm not using, because the alias directs mail to another service altogether) but excludes SMTP. I should be able to send emails to gmail.com addresses using the webmail user interface, but can't using any regular email SMTP client. I could add SMTP for an extra £10.00 per year.

I discovered most of that just before having to leave home for a no-internet weekend, which is why my "I think I've got it" message was so terse.

Workrounds for my send-to-gmail problem are (a) don't send messages to gmail.com addresses, (b) send them with my outlook.com address using smtp-mail.outlook.com, or (c) sign up to pay for my free-for-life email. (It wasn't free, if you think that I worked fairly hard for several years to qualify for it.)

It is a shame that your employment benefit has morphed into a problem.

If your wish is for email to come to your memyselfi@university.net address then my thought regarding a workaround is to send using your outlook.com smtp, but from an identity that is configured with your memyselfi@university.net address as the default reply-to address and with a signature that contains the memyselfi@university.net address. Unfortunately, there is no guarantee that all replies will honor that.

Yes, it's a shame. Graduation perk, rather than benefit from employment!
The other two workrounds are Brian's (use a reply-to: header) and to send messages to gmail.com destinations using webmail. I'll also, when I catch up a bit on the email mess I'm in at present, ask the providers of memyselfi@university.net to consider including an SMTP service for free-for-life emails, now that gmail.com effectively requires it.

I wonder whether entering your memyselfi@university.net address in the "Use an alternate e-mail address for the SMTP envelope" field of your outlook.com SMTP host configuration would work. I think it's a long shot but worth a try.

I'd just about got there, Brian, but I did it the other way round. In Pegasus settings, I'm now trying these.

My Internet e-mail address is: memyselfi@university.net (the one I can't access SMTP for)
Use an alternate e-mail address for the SMTP envelope: memyselfi@outlook.com
SMTP server: the one for outlook.com

That combination appears to be working, or at least not bouncing any of my messages yet. But my test dataset is rather small, growing as I try to catch up on confused and bounced content and time wasted chasing up this and other recent email issues. Thanks for the helpful suggestions!