Not able to send the emails using the "Mail" app on my Mac. The very same issues happen with any 3rd party app, not only the native software. I'm able to view the messages, able to read them, but when I try to send the message the error occurs

Follow all the suggestions in If you can’t send or receive email on your Mac - Apple Support.

Needless to say, if you are using any non-Apple "anti-virus", "cleaning", or "Internet security" products including "banking security", anything at all in that broad category of utterly useless garbage, don't. A lot of things won't work if you do.

The most expedient way to fix that problem is to remove the problematic Mail Account, and then add it again.

First remove: Remove email accounts in Mail on Mac - Apple Support

Then add: Add email accounts in Mail on Mac - Apple Support