Office 365 includes some security defenses that limit the ability of an individual to send huge numbers of messages in a short amount of time. If you have legitimate large mailings to do, please understand these limits and the alternatives you have for delivering your messages.
The maximum number of addresses allowed in the To:, Cc:, and Bcc: fields is 500 per message.
The maximum number of messages that can be sent from your email client is 30 per minute.
The maximum number of recipients that can receive messages sent from your account (or an Exchange Group Account) in a 24-hour period is 10,000.
It doesn't matter whether the recipients are at UM or elsewhere.
The 24-hour period is a rolling window: for example, if you send messages to 2,000 recipients at 8:30 am, then messages to 5,000 recipients at 12 noon, then messages to 2,000 recipients at 2:30 pm, and messages to 1,000 recipients at 4 pm, you won't be able to send any more messages until 8:30 am the next day.
If you try to send a message after exceeding the limit, you will either see an immediate error about the recipient limit, or the message will be returned to you with an error. Save the message to your Drafts folder and try again later.
A message will only be delivered if it can be delivered to all the recipients without going over the 10,000-limit. For example, if you've already sent messages to 9,900 recipients, and you attempt to send one message to 150 recipients, it will not be delivered because it would be 50 recipients over your limit for the 24-hour period.