Question : I recently migrated from exchange server 2003 to exchange 2007, now I have a user who has a 3rd party pop email account he cannot send email from.

I recently migrated from exchange server 2003 to exchange 2007, now I have a user who has a 3rd party pop email account she cannot send email from.

Here is my network lauyout:

SBS 2008 w/exchange server 2007
10 Users
1 user has a 3rd party email account she checks email on and sometimes sends email from. She can receive email on this account but she can not send reply's or emails from this account. Is this an issue with sbs 2008 or an exchange server 2007 security feature?

Answer : I recently migrated from exchange server 2003 to exchange 2007, now I have a user who has a 3rd party pop email account he cannot send email from.

I disagree.  I dont think the email is going through your exchange server.  I belive it is going directly to your gateway (or trying to).  Sometimes gateway routers have outbound firewall rules that limit SMTP traffic (port 25) FROM the LAN to only the exchange server (same reason as AV blocking it).
try these commands:
telenet  exchangeserver 25
ping ISPsmtpSRV
telnet ISPsmtpSRV 25

Do you get a telnet/ping reply?