Many of our client here in Qatar having issues sending mail to some organization like QP , Qatar Petrolium through Barracuda. Please see the resolution here.
Permanent solution 
Upgrade to

Firmware Version 6.X 

‘This is issue has been addressed on that 


  • Fix: Resolved issue with connecting to recipient servers when Enable SMTP over TLS/SSL is enabled (see the ADVANCED > Email Protocol page).
Temporary  solutions
Advanced > Email Protocol and at the bottom change “Enable SMTP over TLS/SSL” to NO. 

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