and I think it's more a problem that you get on a blacklist of the ones who supposed to receive the emails. These are the instructions they sent me .. translated from german :
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General dispatch guidelines:
In order not to risk blacklisting and/or significant dispatch delays on the part of recipient mail servers, it is of enormous importance that dispatch is neatly staggered. In addition to the monthly volume limit, the following limits are mandatory:
maximum 40 mails / minute
maximum 2000 mails / hour
maximum 5000 mails / day