I am trying to get e-mail to a customer of ours. From my internal Exchange server, I can send the e-mial successfully to the customer if there is no attachment. I can send the email successfully to the customer with an attachment (zip, docx, pdf tested) from my Gmail account. If I try to send an e-mail with any attachment (zip, docx, pdf) from my domain, the mail never reaches the customers. All I get from them is a bounce "proxy1.somedomain.com <proxy1.somedomain.com #4.4.7> #SMTP#" (customer domain removed for privacy).

Does anyone have any idea where I can start testing this? It seems to be a problem with attachments from my domain, but why would a server only block e-mail with attachments (any attachment) from my domain only?

Thanks

NOTE: I thought I would add, we have an internal Exchange 2007 server and a Sendmail relay. The sendmail server just takes incoming mail and searches for spam and malware before it relays the mail into my Exhcnage server. The sendmail server doesn't see outgoing mail.