Please increase SMTP timeout when sending email. Per RFC 2821, 4.5.3.2, it should be 5 minutes. It's OK to time out soon if connection is not established, but once it's established, it should wait up to 5 minutes for the 220.

In fact, an artificial delay before sending initial 220 message is one of practical anti-spam measures, and currently SM can't send to the servers that are using it, such as xwall.