I'm having a very strange problem with only one virtual domain hosted on our IMail server. It seems they can receive all email messages but are having problems sending emails with any attachments or signatures. Emails sent without an attachment or signature seem to be sending fine.

This is very weird because I'm not having this issue with any other client on our IMail server.

I think they are using Verizon as their ISP.

Please let me know if anyone can assist.

Thanks!