Yes, it is a personal domain. I assume it is being greylisted?
Greylisting isn't what you think it is it would seem:
Greylisting uses the features designed into email to force the sending server to re-send the email. In the process it takes a tuple of information e.g.:
MTA IP Envelope sender Envelope recipient
and records it.
Wen the delivery is retried, if the information in the tuple matches what's recorded then the delivery is allowed. Some Greylisting will also reject mail that is delivered too soon after the initial attempt.
The idea is that spambots will often not attempt re-delivery, but legitimate mail servers will.