Just because it&#x27;s in the RFC does not mean it will be obeyed.<p>I tried running postgrey - a system that rejects senders it doesn&#x27;t recognise, tells them to &quot;try again in a minute&quot;, and accepts the second email. The theory being that spammers rarely check for these responses because they operate on a &quot;fire and forget&quot; basis. In reality it didn&#x27;t work because lots of email services didn&#x27;t correctly respond to the &quot;try again in a minute&quot; response - which is in the RFC!<p>Maybe it&#x27;s an extreme example. You expect an SMTP server not to be up all the time, you don&#x27;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&#x2F;worry. I found myself default to Gmail a lot of the time for this reason.