Question : the connection was dropped by the remote host

We recently came across a problem sending to two domains.  When we send normal email messages, the messages go through fine to the domains.  However, when we send attachments, the messages sit in the queue, with the message "The connection was dropped by the remote host".  I've checked AV, disabled "outlook autoprotect" in Symantec, I check our IP and domain, we're not blacklisted (we can send regular messages to the domains).  I'm out of ideas.  Thanks.

Answer : the connection was dropped by the remote host

1.Try zipping the attachments and then send them.
2.Check the size of the files that you send is not above the size that your recipient has.
3.Contact with them if they have a firewall or something like that which may be causing this problem.

Hope that helps !