Corporate email is failing intermitently for one user when sending to recipients in other countries from the US

Hello - we have a corporate Exchange 2010 server with about 100 users.  One user is experiencing international email problems for the past couple weeks with email bouncing back to her.  She experiences no problems with domestic recipients, however.  It is also an intermittent issue - sometimes the message to an international recipient will work, while other times it is timing out.  This happens with multiple recipients from different domains, as well.  There appears to be no pattern, as of yet.  there have been multiple failures just this morning.  Not sure how to troubleshoot, but here is the common error between them:

1st MESSAGE:   mfavilli@uol.com.br (mfavilli@uol.com.br) <mailto:mfavilli@uol.com.br>
The server has tried to deliver this message, without success, and has stopped trying. Please try sending this message again. If the problem continues, contact your helpdesk.
#550 4.4.7 QUEUE.Expired; message expired ##

2nd MESSAGE:  contact@qedpackers.com
#554 5.4.4 SMTPSEND.DNS.NonExistentDomain; nonexistent domain ##

3rd MESSAGE:  稲垣慶三 (k.inagaki@lamerco.com) <mailto:k.inagaki@lamerco.com>
The server has tried to deliver this message, without success, and has stopped trying. Please try sending this message again. If the problem continues, contact your helpdesk.

Diagnostic information for administrators:
Generating server: EXCHANGE.lacoinc1.local
k.inagaki@lamerco.com
#550 4.4.7 QUEUE.Expired; message expired ##

4th MESSAGE:  Delivery is delayed to these recipients or groups:

mba@sigma-medical.com.tw

Katherine <mailto:Katherine@sigma-medical.com.tw>

Subject: RE: po#20140521003 please pull in shipping schedule

This message hasn't been delivered yet. Delivery will continue to be attempted.