Mail suddenly not sending outgoing messages

Sometime over the past weekend, the Mail app on my Mac Studio stopped sending messages, randomly affecting certain mailboxes. I have six email addresses connected, all through the same service. The outgoing mail will be diverted to the Outbox where it will sit. Sometimes I will receive an error message stating it can't connect to the server, other times no error message at all (they just sit in the Outbox).

I have tried a myriad list of suggested fixes, including:

• Updating the OS (now to Sequoia 15.4)
• Rebooting machine
• Removing affected accounts and reconnecting them (both manually, and via a config file provided by the hosting service).
• Rebuild the affected account(s), then closing Mail and then reopening.
• Etc., etc.

Occasionally a combination of these steps have eventually alleviated the problem, ie- one step will fix one affected mailbox, another step will fix a different one, and so forth. However for no discernible rhyme of reason it returns. Currently it is affecting a single mailbox, but none of the previous "fixes" are working. Checking the Connection Doctor shows the connection to the SMTP server stuck on "Connecting to server..."

I have tested all accounts via webmail direct through the hosting service, as well as via the Thunderbird app, and they all work fine from those avenues. However, I'd prefer to have this all working from the MacOS Mail app.

I've tried to be as concise as possible, but if any further clarification is needed, please feel free to ask.

Thanks in advance for any assistance.