Here's a good one that most people do not know about.

We recently moved into a new data centre with an outsourcing provider who implemented a firewall between the data tier (where all the SQL Servers are) and the application tier (where the Exchange server is).

The upshot is that although the appropriate holes are available in the firewall, if an sql server does not send an email for 1 hour, then the firewall closes the connection.

To get round this we have had to set up a job that does an xp_Stopmail and an xp_Startmail.

Hope this helps