Specs about mail sending

If the Messaging service fails to connect/login to the set SMTP server when sending an e-mail the first time, does it give up then or does it try again later?

Sometimes I get the following error:
Cannot send email. Invalid email settings

But then without changing any code or the settings sometimes mails are sent correctly. I just received an e-mail which I think was delayed, this leading me to believe your system does have already some kind of re-send attempt mechanism in place.

same strange situation here ?"!