I asked Comcast about their email policies. Their response:
"There is a maximum number of recipients that you can have in the To:, Cc:, and Bcc: fields, a maximum number of times you can click Send per minute, and a maximum amount of messages you can send at one time. Once our mail server detects that multiple addresses in your To: field do not exist, the e-mail will not be sent to any other recipients. Due to security concerns, we cannot reveal the actual limitations."
While I can understand their reluctance to reveal this information, it puts legitimate customers in the position of not knowing exactly what they've purchased. Not to mention that any would-be spammer could simply test their system and determine the exact limitations.
It also puts customers in a Catch 22 situation. Do you send each message immediately, in which case you risk violating the number of sends per minute? Or do you queue messages, in which case you risk violating the max number to send at one time?
And how many names can you include on mailing lists? What about folks who have no email, who are on the list so that everyone who does have email can see the whole list?