READ OVER BEFORE SENDING and EDIT as NEEDED:
Under Microsoft's terms of use, users cannot use their email accounts to send bulk emails.
I suggest you set up a bulk mailing service for this purpose. We use MailerLite. It's simple to use and inexpensive.
[We are about to release the mailbox. It should take about an hour (or less, hopefully) on Microsoft's end to get it unblocked. ]
The reason we have to be very thorough before enabling the mailbox again is that if the same issue occurs again, the next time it happens Microsoft will require us to call in to get it resolved, and to provide proof to them that it is really resolved this time. So we have to be very careful and make sure it's handled correctly so there are no future problems.
In a nutshell, if your users continue to use their email accounts for sending bulk mails, we can't guarantee this won't happen again, and it will take longer to resolve next time.
Please understand these are not our rules. They are Microsoft's rules. We have to play by their rules, just as the end users do.
Thank you for understanding.
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IF THEY PUSH BACK ("what constitutes bulk mail?" "How many email addresses?" We’ve sent emails like this before...") . READ OVER BEFORE SENDING and EDIT as NEEDED:
In answer to your question, Microsoft may have let some emails through in the past but that is no guarantee they will go through in the future. It's possible there was something that Microsoft flagged as particularly "spammy" in this email such as the subject or something in the body of the email. However, it wouldn't be relevant to any future emails you try to send because if you try to send to this many users, there's simply no way to guarantee you won't be blocked again. So whatever the reason for this particular email not going through, I strongly suggest you stop using your mail accounts this way.
I suggest not sending mail to more than 20 users at a time. (And if you send 20 and then send 20 more a few minutes later, etc., Microsoft may still flag it.)
Again, this is all under Microsoft's control, not ours. I think they are deliberately a little vague in their terms of service so that they can adjust the criteria as necessary.
Here is a link to MS terms of service, FYI:
https://www.microsoft.com/en-us/legal/terms-of-use
Spam is a huge problem, as I'm sure you're aware, and most mail providers are doing their best to stay ahead of it and keep their servers and their servers' reputations clean.
It's simply not good practice to use your email address to send bulk email. Many of those emails are probably ending up in spam folders. In the end, using a bulk mail service will look more professional and give you the reliability you are looking for.