I think I solved it.

I installed Thunderbird to see whether I could send mail using a different client program. It worked, I could. However, when Thunderbird automatically set itself up for sending and receiving mail I noticed that it chose a mail server with a different name to the one I've always used in Pegasus. ****-****.accessdomain.com instead of ******.gridserver.com. So out of curiosity I tried changing the server name in Pegasus, and it now sends mail to an address it did not previously send to.

I haven't checked it throroughly yet but fingers crossed this has fixed it.

Hope someone can shed light on this. When I use PM to send mail to certain recipients they don't receive it. It does not appear in their spam folder and I receive no notification that the message was not delivered, it just simply disappears. This began about 12 months ago with one or two addresses, but now several are affected. The affected recipients work in different organisations so it's not something specific to a particular mail server on the receiving side.
If I send emails to those addresses using my domain's webmail service they always get through, so the issue is something to do with Pegasus, it's not my email address being ID'ed as spam. I just upgraded to the most recent version but the problem persists.

Suggestions...?

[quote user="Chris Dancer"]
If I send emails to those addresses using my domain's webmail service they always get through, so the issue is something to do with Pegasus, it's not my email address being ID'ed as spam. I just upgraded to the most recent version but the problem persists.[/quote]

I doubt it very much. Really, I don't know why some members of this community insists that this or that happened to their mail is Pegasus Mail fault. It is not! Note that Pegasus Mail is a client to a given e-mail service and so most of do's and done's are on server's side. I'd recommend you to turn on a session log so you can follow what's happening to your messages when Pegasus Mail delivers it to the server. This will clarify what happens between Pegasus Mail and your server, but not what may be happening to your recipient's servers and clients.

[quote user="Chris Dancer"]Suggestions...?
[/quote]

Besides that already given you should consider the action of third-party tools like antivirus, your server's spam and/or malware filters, etc, but I'm quite confident the session log will tell you what may be going wrong.

Even though they go (or should go) to different servers via different routes it is still possible that they get caught in/by the same swamp of pseudo filtering based on some list, rule or another, all to preasure the use mail (read Web Mail) that will share more of our data for Analysis "to better serve us"

I see the same effect in (I hope) a limited number of calls, TTBOMK only when

I client (who uses BTInternet) emails me from a smart phone, if I reply without changing the To: address to the DList I use for this client the mail disapears without trace, sys logs show nothing odd

For a while we tried to get BT to sort the problem but they claimed that my mail lacked some specific three letter Acronym (S something something IIRC) and they could not accept mail without it, oddly if I start a post I can do that without (AFAIK) issues

Thank you, I wasn't blaming Pegasus mail, just pointing out that there is something specific about using it that is causing the trouble somewhere down the line.

I didn't know about session logs. I've created some and the only thing that looks vaguely suspicious to my untrained eye is that every session ends with this warning:

23:35:38.817: << QUIT<cr><lf>
23:35:38.973: >> 221 n13.c04.mtsvc.net closing connection<cr><lf>
23:35:38.988: [!] Warning [Benign]: SSL connection improperly closed by remote host.
23:35:38.988: --- Connection closed at 19 Feb 2020, 23:35:38.988. ---
23:35:38.988: 

It seems to have gotten so bad now that I cannot send emails to any address from Pegasus.

[quote user="Chris Dancer"]

Thank you, I wasn't blaming Pegasus mail, just pointing out that there is something specific about using it that is causing the trouble somewhere down the line.[/quote]

I'm sure you don't. The problem when one says "...Pegasus does something wrong..." is that it may give a wrong impression to others. Pegasus Mail is the most dependable email client by far.

[quote user="Chris Dancer"]I didn't know about session logs. I've created some and the only thing that looks vaguely suspicious to my untrained eye is that every session ends with this warning:

23:35:38.817: << QUIT<cr><lf>
23:35:38.973: >> 221 n13.c04.mtsvc.net closing connection<cr><lf>
23:35:38.988: [!] Warning [Benign]: SSL connection improperly closed by remote host.
23:35:38.988: --- Connection closed at 19 Feb 2020, 23:35:38.988. ---
23:35:38.988: 

It seems to have gotten so bad now that I cannot send emails to any address from Pegasus.

[/quote]

Session logs are important to pinpoint any wrongdoings about your settings, communication protocols, login, passwords, etc. If you didn't notice nothing important on any of those logs is because your mail was properly sent and accepted by the server host. From this point and beyond it's on server's hands. The warning you got is benign as said.

You can post the entire session log here taking care to remove or obfuscate those lines containing your login and password or any other sensitive data.

Have you tried any other mail service? You didn't say much about your mail provider. Maybe it is just a matter to change ports and security protocols.