Who runs an SMTP server from a dynamic address?
Almost all SMTP traffic coming from dynamic addresses is SMTP traffic caused by worms and similar malicious programs. Typically, this means they are either sending copies of themselves or working as zombies to send spam. Because it is likely that virtually no legitimate traffic will be sent this way, many hosts block DUL SMTP servers traffic, quite likely, yours already does (unless you are using my services, and then yours will shortly).
