I found 3 messages in the 'Deferred' section of the Mail Queues all going to the same address. However, I can't seem to find any good information on the message that it's showing.

Code:
lost connection with mail.domain.com[xxx.xxx.xxx.xxx] while sending RCPT TO
I've ran DNS checks on the domain and everything seems to checkout.

Where do you think I should start looking?