Outgoing mail to some recipients disappearing

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...?