what kind of email accounts are effected? If you send you get an error message (?) - if yes, which? Which Mail program are you using/did you try with a different program/as a different user? What have you tried so far? Please provide more detailed information.
OK. I am using mail.app. The accounts include my iCloud account & other IMAP accounts on different hosts. It applies to all my accounts in mail.app. Restarting mail has no effect