I tried running postgrey - a system that rejects senders it doesn't recognise, tells them to "try again in a minute", and accepts the second email. The theory being that spammers rarely check for these responses because they operate on a "fire and forget" basis. In reality it didn't work because lots of email services didn't correctly respond to the "try again in a minute" response - which is in the RFC!

Maybe it's an extreme example. You expect an SMTP server not to be up all the time, you don't expect them to randomly reject mail and ask you to try again later, but you get my point. It just adds to the stress/worry. I found myself default to Gmail a lot of the time for this reason.