Mail Stuck In Outbound Queue

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Looking at my outbound mail queue I can see a few e-mails all queuing up for
one e-mail address i.e.xx@xxxxxxxxxxxx)

E-mail to all other addresses go without problems and I can sucessfully send
e-mail to xx@xxxxxxxxxxxx from other accounts (Hotmail, for instance).
xx@xxxxxxxxxxxx is also receiving e-mail from anyone else without problems.

We e-mail this address quite a lot and the problems seemed to start
yesterday and we've done nothing to the server at our end.

I'm running SBS 2003 R2 Standard, fully patched. GFI's Mail Essentials and
Mail Security are also installed.

Can anyone point me in the right direction on this?

Waggers.


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