Sendmail    451 4.4.1 reply: read error from     Deferred: Connection timed out with

I am getting the following error with some outside connections:

220 Ready
>>> EHLO mail.scop.net
250-ESMTP Server Ready
250-SIZE 104857600
250-DSN
250-STARTTLS
250 TLS
>>> STARTTLS
220 Server ready Ready to start TLS
>>> EHLO mail.scop.net
250-ESMTP Server Ready
250-SIZE 104857600
250 DSN
>>> MAIL From:<dmartinez@scop.net> SIZE=22877
250 +OK Sender OK
>>> RCPT To:<JARoberts@ironmountain.com> NOTIFY=SUCCESS,FAILURE,DELAY
250 +OK Recipient OK
>>> DATA
354 Start mail input, end with "<CR><LF>.<CR><LF>"  
>>> .
<JARoberts@ironmountain.com>... Connecting to colsmtp02.ironmtn.com. via esmtp...
<JARoberts@ironmountain.com>... Closing connection to nussmtp01.ironmtn.com.
220 Ready
>>> EHLO mail.scop.net
250-ESMTP Server Ready
250-SIZE 104857600
250-DSN
250-STARTTLS
250 TLS
>>> STARTTLS
220 Server ready Ready to start TLS
>>> EHLO mail.scop.net
250-ESMTP Server Ready
250-SIZE 104857600
250 DSN
>>> MAIL From:<dmartinez@scop.net> SIZE=22877
250 +OK Sender OK
>>> RCPT To:<JARoberts@ironmountain.com> NOTIFY=SUCCESS,FAILURE,DELAY
250 +OK Recipient OK
>>> DATA
354 Start mail input, end with "<CR><LF>.<CR><LF>"  
>>> .
<JARoberts@ironmountain.com>... Deferred
Closing connection to colsmtp02.ironmtn.com.

Not all users are experiencing this issue and only seems to affect some outside SMTP connections.  I had to rebuild our Mail Server because of hardware problems so I did a fresh build on new hardware.  We are on Linux RH 8.0.  I can send mail to other SMTP's and here is a weird twist to the problem.  I have  a user with email address of first name.last name with an alias of first initial.lastname.  She cannot send to a specific e-mail she gets the same error stating deferred connection timed out.  But I can successfully send to that address with no problems.  What seems to be the issue.  I have looked at the MTU, but if some emails are being sent out I figured that could not be the issue.  Also, we are receiving fine with no problems.