There are many reasons why an email may not be delivered, and most of them have nothing to do with what mail client you are using.

The delivery failure messages you are receiving should contain the reason why the mail was not delivered.

If you can provide some more information (copies of the various failure messages would be a good start) then the reason for these failures could be narrowed down a bit.

The problem occurs sporadically. I can send messages sometimes and they go through just fine. Then a few e-mails later they are returned as mail delivery failures. It doesn't seem to matter if it is a single destination or using the reply or forward buttons, some go through, others are returned. I have just switched to a broadband server called Wild Blue and their folks don't know much of anything about Pegasus and don't support it yet, they say.

Hoping for some insights. Thanks.