I don't know if I'd do all of them at once but maybe you could Cc: or Bcc: in batches (maybe 50 would be OK?).
As for getting banned, I imagine that depends on if it's SPAM and anyone reports it. But it might be a good idea to look at the TOS or even ask in the support forums just to be safe.
Although IMHO it would be best to just post the message as a status update and let them see it that way.