Cannot send mail outside of domain

What does the non-delivery reports say? Are you using yourself as mail server (based on MX) or you're using a smart host?

Check for available disk space
exchange does this if there is insufficient space

1) Make  sure your IP Address is not blacklisted , check the same using following url.

http://mxtoolbox.com/blacklists.aspx

2) Check Queue viewer Under Toolbox and check the reason if e-mail stuck on the same.

3) Try to send e-mail to other domain using Telnet and check the results.

@ cpmcomputers : Sir, Internal Mail flow is working fine so there will be no issues with disk space.

Exchange server enters back pressure mode when available disk space drops to around 2%.

@ cpmcomputers : Sir, ,Yes that's true , If exchange server found there is a back pressure mode that time exchange server will stop transport service and will not allow internal or external mailflow until the  disk space has drops to normal .
Please don't mind.

It has three modes
Normal medium and high
In medium internal works but not external
(If this is the problem it will be reported in the event viewer )
Will take seconds to verify or eliminate it S a cause ?

If it's not disc space
I would looking next at either the receive connector or a DNS resolution issue
@viral is correct in suggesting looking at what is happening in the queues

As the issue is with sending, I assume you can receive email from all other domains (i.e. yahoo.com, outlook.com, etc.).  In this case then you need to look at your send connector(s).  Your send connector may be configured to relay messages to gmail.com but no other external domains.  Your send connector should be configured to route email to all domains (i.e. *)

check your ip or domain if is Blacklisted
check your  Connection Filtering or Port 25 blocked at the remote site
check this link :

http://blogs.technet.com/b/sbs/archive/2008/01/03/what-to-check-when-exchange-cannot-send-email-to-certain-domains.aspx