After encountering a failure in a service that passed on emails to a mail server, my first point of investigation was to ensure the mailserver itself was functioning correctly.
So first things first, fire up your command shell window. Then start typing:
telnet Mailserver 25
>220 ServerName.domain Microsoft ESMPT MAIL Service, Version x.x.x.x ready at [time]
>250 ServerName.domain.com Hello [IP]
mail from: email@example.com
>250 2.1.0 firstname.lastname@example.org....Sender OK
>250 215 email@example.com
>354 Start mail input; end with <CRLF>.<CRLF>
. <–Press the dot button
>250 2.6.0. <ServerNameaaaAAA000bbbbbb111.domain.com> Queued mail for delivery
And, hey presto! I got an email.
This determined that the culprit wasn’t the mail server, and I learned something new in the process. More info can be found at http://www.yuki-onna.co.uk/email/smtp.html