Particular destined Mail stuck in the queue, but not all the time

Hello.

We have Exchange Server 2003 SP2 on Windows Server 2003 and
are having some weird problem.

The problem is some e-mails which are sent by our user in the organization
stuck in the queue. But not all the time. Most of the time, the user's mails
go thru. However, some mails are just stuck in the queue (all same
destination
- our customer).

I have traced his mail on the ESM and found out that those mails which are
stuck in the queue are stopped at "SMTP: Message Routed and Queued for
Remote Delivery".
Usually the log says "Message transferred to ***.***.com through SMTP" when
mails go out
without any problem.

The mails which are stuck in the queue eventually time-out & Exchange
generates an NDR
with SMTP error 4.4.7 says "Could not deliver the message in the time limit
specified".

We have checked the other end e-mail server for the log tho, there was
nothing.
that means the mail server at the other end never had a connection with our
Exchange Server for those particular mails.

Funny thing is particular users' mails destined to particular domains are
stuck, but not all the time.

On the Exchange server, MX records of the other end mail servers resolve
just fine & I can telnet to those server without any problem (ran whole
command to
send mail).

Does anyone have any idea!?

Need a help!!!

Thank you very much in advance.

Regards,

Takeshi