Mass mail of any kind is prohibited through the Pobox outbound mail relays. ISPs collect spam complaints based on the server that sends the mail, not the person, so if your mass message triggers a spam complaint, you could be preventing all Pobox customers from sending mail to their individual correspondents.
Before you say, "but my message is not spam!", here are some messages we have seen reported as spam in the past:
Clearly, none of these messages are spam, in the sense that none of them are trying to sell a product. However, all of them were mass messages, sent to a CCed or BCCed list of people. For this reason, there are no exceptions to this policy, and we do not provide information about our outbound message limits.
If you need to send mass mail, please use a mailing list or email marketing service. Not only do they include all kinds of tools to make managing a list of addresses easy, they also have tools to manage spam complaints. So, they can make sure that the person who reported your last message as spam doesn't get the next one, which means all the other people on your list who do want to receive your mail will receive future messages.
For non-commercial use, Google Groups and Yahoo Groups provide free distribution services. For email marketing, we're fans of our sister service, Listbox.com, but Constant Contact is a popular option, too.