@davewalker
You should use the G one going forward. Back up the mailbox of the @ one and delete it. The error is probably due to Mac Mail trying to use the SMTP connection in that old @ account. There is a dropdown in the compose view by which you can pick with which SMTP connection you want to send your message.
I see some people out there saying “how hard is Mastodon? it’s like email!” and I can’t help but think “do you realize how hard email is”
Email involves 1 protocol (SMTP) for sending and receiving, 2 protocols (IMAP or POP3) for client connection, TLS versions of them, and an undocumented semi-structural data format (mbox), which is not fully compatible across clients. Formatting is supported only by agreement of big client vendors on an old subset of HTML.
Don’t blame yourself for failing to comprehend this mess. It’s something as legacy as mainframes that we can never replace.