We limit outbound mail from the SMTP server by both volume (mass mail) and content (spam). These limits are set in response to feedback from ISPs regarding what they will and won't accept from us. As such, we reserve the right to adjust them when and as needed, and we do not publish the limits.
Why does Pobox filter mail for paid accounts?
Spammers are relentless in their attempts to send spam. Sending mail from servers with a good reputation is the best way to get your message into people's Inboxes. Over the years, we have placed increasingly stringent limits on trial accounts to keep spammers from abusing our reputation. So, at this point, the most common source of spam from Pobox's SMTP service is legitimate, often longtime, paid accounts, whose accounts have been compromised. The methods for this range from a bot or virus getting installed on a customer's computer, to their password being (rarely) guessed or (more frequently) gained through a security breach on another service, to simply abuse of an account by someone other than the account holder. Following high-profile security breaches like Gawker and Sony, we have seen as many as 100 accounts compromised and used to send spam. As a result, paid accounts must still be monitored and occasionally suspended.
For the same reason, ISPs do not draw distinctions between legitimate mass mail and spam. If you are sending mass mail, and you do not follow email "best practices", they treat your mail as spam. Even a small number of "mass" messages a day can quickly tip the ratio for our servers from one-to-one mail to mass mail. In order to insure all Pobox customers have their mail delivered to their correspondents' Inboxes in a timely manner, we follow the restrictions on mass mail as provided to us by large providers like Gmail.