Office 365: Do not use Exchange for mass mailings
As a whole, Office 365 is not designed to be used for mass mailings. Microsoft's outbound spam filtering monitors and takes action on outbound senders in order to prevent compromised accounts from sending phishing or spam emails. We do not have much insight into the exact limitations Microsoft places on outbound senders--we can tell you that they both look at number of recipients and amount of total email sent within a given time.
If Microsoft's outbound spam detection is tripped, the sender is placed on a blocked senders list. We are able to remove blocked senders from this list, but if a sender gets blocked too many times in a certain period of time, we would need to open a support ticket with Microsoft to have the sender unblocked.
| 550, 5.1.8, Access Denied, Bad outbound sender
Your message couldn't be delivered because you weren't recognized as a valid sender. The most common reason for this is that your email address is suspected of sending spam and it's no longer allowed to send messages outside of your organization.
If you plan on routinely sending bulk messages, you may want to look into a bulk email service. The University Development Office runs a service called Eloqua that may be appropriate for your mass mailing: https://wikis.utexas.edu/pages/viewpage.action?spaceKey=eloqua&title=Eloqua
If you are only sending to internal Exchange users, you can create an Exchange distribution group and add the intended recipients as a member of the list. Sending email to a distribution group in Exchange gets around the recipient limits. A distribution group counts as only one recipient regardless of how many people it contains.
If you are going to be sending several hundred messages to several hundred recipients, we suggest that you break up your list into separate lists and send the messages throughout the day. If you do end up on the blocked senders list, reach out directly to firstname.lastname@example.org and we will be able to assist.