Can't send certain messages from Pegasus. Due to ISP increased security measure?

I've been happily using Pegasus Mail since 1991, safely moving all my email folders from computer to computer over the course of 30 years.  I would hate to have to stop using Pegasus!

But suddenly I can no longer send certain kinds of messages despite using the same ISP and internet settings inside Pegasus which I have been using for the last five years, and my ISP has not altered its main settings.

Maybe the writing was on the wall when about a month ago my ISP sent me a message saying they would be updating their security for logging in, authentication, SSL, or something like that, warning that out-of-date email clients may need to be upgraded.  I looked to see if there was an update for "Pegasus Mail" but there hasn't been.  I checked my ISP's supported email clients and email apps and "Pegasus Mail" is not among them.  So I am guessing that there is something about certain emails sent from Pegasus which the new security arrangements do not like.

I am having no problem sending fresh messages, i.e. I write a fresh email, even include a CC address and a BCC address.  It goes.

I am having no problem replying to somebody else's fresh email to me.

But I have not been able to send an email addressed to three Pegasus Mail distribution lists, for example to addresses like "LIST1273.PML"  I receive a message saying "SMTP Network or Protocol Error sending email".  My only option to get that urgent message out to the people on the three lists was to write a fresh email within the ISP's website.

And I have not been able to reply to an email that has been replied to three or four times by myself and a correspondent.  In this case I receive a message referring to "pipe-lining".  Again, my only option to get that message out was to copy the text out of the rejected email and create a new email within the ISP's website.

I have done a bit of hunting on Google and found that some ISPs offer a settings button named "Allow less secure apps", but my ISP doesn't provide this option.

I'd hate to lose use of Pegasus Mail.  Any ideas, please?

Thanks.

Cat009.