I've been helping a friend who uses Telstra/Bigpond (Australia's largest telco) as her mail server. For some time her MacBook Pro has been able to receive new mail but not send or reply - it just returns a server error message. When she uses her iPad(s) and iPhone, receiving AND sending/replying works fine. All the mail server settings on her Mac are correct. Telstra support was not able to resolve the issue, concluding that "…we have lots of problems with Apple Mail on Mac's…"
We recreated her account on my MacBook Air and it works fine!
We downloaded Spark (
https://apps.apple.com/app/apple-store/id1176895641 ) as part of the problem resolution process.
It works fine! She can now send and receive!
I don't understand why the problem existed on her Mac but not on mine. Corrupted file perhaps. But it was not something we could fix.
Changing to an alternative email client was easy and solved the problem!
Stay safe,
Peter