horde relay error

[harty83 at gmail.com]

I'm using horde with kolab and I have several identities set up within
horde.  But when I try to send an email I get an error that says:

"There was an error sending your message: Failed to add recipient:
alan.hartless at gmail.com [SMTP: Invalid response code received from server
(code: 553, response: sorry, that domain isn't allowed to be relayed thru
this MTA (#5.7.1))]"

Is kolab or gmail not allowing the relay?  How do I fix it?  I send mail
with different identities through my kolab server when I use kmail so I'm
not for sure why horde won't work.

Thanks!
Alan
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