Mail suddenly stopped sending outgoing emails!

You likely have corrupted settings for your "Outgoing Mail Server (SMTP)". Open Mail>go to Mail's Preferences>Accounts>there set up your Outgoing Mail Server which will INCLUDE resetting up your Password, as its asking you to. Click on "SMTP Outgoing Mail Server" near the bottom of the Accounts window while your mail account is highlighted in the left hand column. When you click on SMTP... you will see a drop down box icon offered at the right side of the line. Click it and then select at the bottom of the the list "Edit SMTP Mail Server". Fill in the required settings for your ISP mail account using their recommended settings.