I recently was forced to install Pegasus on a new machine, following which I set Pegasus up as the default mailto app. This, however, has triggered a hard block by my security program, PCMatic, who have marked the wsendto file as BAD. I then contacted PCMatic, who cheerfully told me to forward a copy of the wsendto to them for their verification and addition to their database of good files. I set about doing this, (using Pegasus), attached a copy of the wsendto file, and sent it off, only to get a rejection of the message:

Delivery has failed on the enclosed message for the following
reasons reported either by the mail delivery system on the mail
relay host or by the local TCP/IP transport module:

554 5.7.0 Reject, id=07340-05 - BANNED:
application/octet-stream,.exe,.exe-ms,wsendto.exe

I think it highly unlikely that there is in fact the slightest issue with the sendto file itself, but there seem to be an awful lot of agents out there who disagree. Any advice on how best to resolve this? (Erase and re-install Pegasus, to get a clean copy of the wsendto file, in case my copy really has been altered by some evil actor? Go after my mail provider? What else??)

It is common for attachments that are executable files to be blocked by email servers. You can try zipping it and then sending the .zip. It may still get blocked if the server doing the blocking also looks in zipped attachments.

I am surprised that you can't whitelist it on your PC and that PCMatic does not provide a submission method other than email.

Executable and most other kinds of binary files are always blocked (despite of not knowing what they actually do) by email providers, you can't get around this other than these "security"-providers offer a special way for submission. The only thing you can do is to configure this "security" scanner's black or white list (whatever they use) so that it ignores the file from being scanned by them at all.

WSendTo is a fixed part of the Pegasus Mail package which comes in a signed installer that would be detected as broken in case anyone tampered with it even by MS' own signature checks without having any external software doing anything else other than annoying users with false alarms. WSendTo is digitally signed as well, BTW, just in case you would care to check (right click > Properties > Digital Signatures). Here's also a tool for checking digital signatures: https://learn.microsoft.com/en-us/sysinternals/downloads/sigcheck.

Issue resolved, (at least partially). PCMatic added the file to their whitelist. (I could not do so myself on my own local whitelist, but they were quite happy to check it out themselves and confirm that it is safe.) Thank you all very much.

The reason I said it was "partially" resolved is that now that the file is permitted to run, Windows is complaining - it says "Mail Client not found: Could not perform this operation because the default mail client is not properly installed". I think this is probably because I have Pegasus installed on a secondary drive, (drive D), and Windows is looking at C: I haven't taken the time to go back and try to figure it out.

What exactly did you do when you got this error message? In a browser you may need to set WsendTo explicitely as MailTo handler as well. And assign it to the mailto protocol as the system's email handler (although setting Pegasus Mail as system mailer in Pegasus Mail's options should do this for you on Tools > Options > Hyperlinks). If you were using the Windows "Sendto" option (via file Explorer's context menu) this might not suffice, though, just let us know.