I have a problem - any outgoing calendar invitations in outlook getting a 553 relaying disallowed error. 

I have two accounts defined in outlook and one of them is in zoho mail and the other is an old account, not sure where it is hosted.

Any time I try to send out an invitation it fails and says 553 relaying disallowed to <old account>.

Even if I change the sender it still says that.

When sending regular email - I have no problem sending through the new or old accounts. does not matter. 

What can I do?

I thought of changing the zoho account to be the main account but it seems to be the case. The other one does not have a pst file defined (if that matters).

I would be grateful for any and all input.

Thanks!

Tal