Email Sender Callouts are bad
As an anti-spam measure some emails servers will do a callout on the sender's email. The receiving server will check with the sender's server to verify that the email address of the sender exists on the sending server. This method has become unreliable in spam filtering as spammers are quite aware of this technique and will forge the from address to be a real email address. Because this no longer works very well at preventing spam, what this method does do is tax the server resources of both the originating server for the forged email address and receiving server. If you receive a lot of spam messages this could be quite a significant overhead for both servers. Sender callouts of this nature are now looked down upon and suggested that they are disabled.