Free hotmail accounts, at least for a while, had a "ten messages per day" sending limit, after which hotmail blocked me from sending anything for the next 24 hours. (Mine wasn't a "free account" and they did remove the block when I complained.)

Some providers also have made a practice of blocking all mail from a sender if a message (or a few together) are diverted as "bulk mail." A block of this kind commonly is for a fairly short time, and should go away - for individually addressed messages - if it's not re-triggered.

I had one recipient a while back whose service blocked anything from hotmail, unless the sender was in the recipeints online address book. One message would block for ten days, but a second message received before the ten days expired resulted in a permanent block that even the recipient couldn't remove. He got a different service quite quickly. In this case, with a little research, I found the service had a "published policy" that explained this "feature." Since such blocks are usually intended as anti-spam devices, most services won't tell you what they are doing so it can be difficult to resolve one of these "quirks."

My recollection is that the last time I tried to look at aol policies, you had to log in with "your aol address and password" even to look at their FAQ page; but that may have changed as it's been some time ago.

John