The reason SMTP traffic won't work with a regular HTTP proxy, is because well.. The obvious. SMTP isn't HTTP traffic, it's a different protocol, and operates outside of 80/8080, typically 25/465.
Hope that gives you some clarification as to why you need a SOCKS vs an HTTP proxy.
EDIT**
Realized i didn't answer the question regarding the relationship. The reason being that the outgoing IP address is included in the header, so if an SMTP/IMAP/POP3 server sees 100k requests from the same IP.. There if clearly something wrong going on.
Alternatively, if your using 1000 gmail accounts to game the system and avoid filters, Google is going to ban your ass quick time if you're connecting to 1000 accounts with a single IP. Thus the need for proxies. You typically want 3-5 accounts per IP for using the major providers such as Google Yahoo and hotmail.
