Mail not sending emails?

I am getting my emails, but when I send reply emails or new messages, the emails don't seem to be sent. I am using gmail. Does anybody know what I should do?

check the outgoing mail server in your mail preference's. make sure they are same as the gmail servers.

I have the same problem. Some ISP's don't allow e-mails to be sent via IMAP and there is nothing you can do. This makes Mail all but unusable. I think it has to do with spamming issues.