My email uses greylisting which is where the first email received from a server gets a "busy" response - the idea being that spammers just fire and forget whereas real mailers will retry.

Unfortunately, some senders take so long to resend that it's timed out. The second time will work though. Unless they have multiple servers. Some have so many servers that you have to do this a multitude of times until you lose the will to login or forget what you were going to do anyway.