You are welcome. I wish it had solved your problem.
What type of mail account is it? I've read other people are having trouble with Gmail accounts. If it is a Gmail account, you might want to check the Gmail website. I've seen links posted to a discussion there.
There has been at least one case where the authentication for the SMTP server "got lost". I'm neither saying it was the OS, nor am I pointing at the user, but fact is it went missing. Fact is also it's a bit hidden and can easily be overlooked.
In Mail -> Preferences -> Accounts -> "your problematic account" -> Account Information tab, click onto the the entry next to Outgoing Mail Server (SMTP), then onto Edit SMTP Server List. Make sure the correct server for the account is selected, then cllck on Advanced. Other than a quick check of port and SSL, make sure that the correct authentication is selected. For most servers it's Password, and the user name and password must be entered.