Question : trouble sending email to particular domains

Kind of an Exchange novice.  A few days ago one of my users had email to two domains returned to him.  I had another user today with the same issue.  If I track the email through message tracking the last message I see is:

"message transferred to postini through smtp"

Part of the returned email error is:

"Could not deliver the message in the time limit specified. Please retry or contact your administrator.

"

We have a third party doing our spam filtering, but they've told us they are doing nothing with outbound mail.  I did a bit of research and learned that Postini is a spam filtering product.  It also looks like error 4.4.7 is an indication of a problem with the receiving server.  Do the above messages/errors mean my users mail is reaching the remote mail server and then being rejected?  

Thank you.

Answer : trouble sending email to particular domains

oh.. so your sending email to someone who is using postini for their 3rd party spam filtering.

If postini accepted it, I doubt there is any more you can do.  Unless your an open relay or postini thinks your a spammer for some other reason.??

if postini accepted it, you know your server resolved the mx record, connected, sent, and got a ACK from the server listed on the mx record.

I would email the receipient postmaster with the logs showing this, and let him deal with it.