Can't send email through SMTP server

Question

As some others have reported, I recently started having problems sending emails (most of the time) through my regular smtp server (receiving is not an issue). Once in a while it does go out, but most of the time I get a message "Can't send message using the server smtp.XXXX.com". I know it's not the smtp server as I can go out with my browser and use webmail and it sends fine, using the same server.
It only affects this one main email account - my other accounts seem fine (including gmail, mac, and another pop accounts), at least so far.
It is extremely frustrating - saw a whole lot of ideas for fixing it (changing ports, etc.), but don't want to mess up all the accounts that are working. I'm a long time Mac user, but not a real techie.

Answer 1

I know it's not the smtp server as I can go out with my browser
and use webmail and it sends fine, using the same server.
This is wrong. Whether you can send via webmail has nothing to do with whether you can send using a standard mail client such as Mail. The outgoing server that ends up delivering the message may be the same in both cases, but the outgoing server you communicate with is not. Actually, when sending via webmail you don’t even use SMTP to communicate with the outgoing server.
There are circumstances under which you may not be able to send using a standard mail client such as Mail no matter what you do (e.g. because the ports needed for SMTP are blocked by the ISP at the location you’re trying to send from), whereas webmail works fine — and may be your only option in those cases.
If it works sometimes, that means your account settings are probably correct, but you cannot discard that there is a problem with the outgoing (SMTP) server that Mail has to communicate with, even if sending via webmail works fine.
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That said, go to Apple Menu > System Preferences > Network, choose Network Port Configurations from the Show popup menu, and make sure that the configuration used to connect to Internet appears at the top of the list. Leave checked (enabled) only the port configuration needed to connect to Internet and Built-in Ethernet (in that order if not the same), uncheck (disable) the rest of network port configurations and see whether that helps — if it doesn’t, turn ON again the ones you want enabled.
Try using a different method to connect to Internet, if possible, or connecting the computer to Internet as directly as possible (i.e. bypassing any routers that might be present, using an ethernet cable instead of wireless, etc.), or shutting down both the computer and the router/modem used to connect to Internet, restarting, and see whether that makes a difference.
Mail keeps information about outgoing (SMTP) servers in a separate list independently of the mail accounts themselves. The account settings just associate one of the available outgoing servers with each account. Orphaned or dangling outgoing server entries (i.e. not associated with any account) sometimes cause weird sending problems.
Go to Mail > Preferences > Accounts > Account Information > Outgoing Mail Server (SMTP), choose Edit Server List from the popup menu, and delete any servers that shouldn’t be there — the Edit Server List panel shows the account each outgoing server is associated with.
If the problem persists and you have more than one mail account, try moving the account that has the problem to the top of the list in Preferences > Accounts.