If you are sending a large number of emails (or a large email) to recipients at Virginia Tech (@vt.edu), you need to be aware of the following restrictions in place:
Each of these items is explained in detail below.
Recipient Limit: 500
Message Size Limit: 150mb
Recipient Rate Limit: 10k/day
Message Rate Limit: 30/minute
Recipient Limit = the number of unique users sent to per email message. Within Exchange, Distribution Groups count as a single user. Exceeding the limit causes the entire message to be rejected with a bounceback NDR to the sender.
Message Size Limit = maximum size (including attachment) of an individual email message. Messages exceeding the size limit will be rejected with a bounceback NDR to the sender.
Recipient Rate Limit = maximum number of unique users sent to in a 24 hour period. Exceeding the limit causes any messages above the limit to be rejected with a bounceback NDR until the rate limit rolls over.
Message Rate Limit = maximum number of email messages the server will accept within 1 minute. Within Exchange, sending more emails than the limit has the surplus emails sitting in Outbox that drain over time. No rejections or bouncebacks, just delayed deliveries.
(If you programmatically sent 80 emails from Exchange Online, the first 30 would immediately send with the remaining 50 sitting in the Outbox. After 60 seconds another 30 messages would deliver out of Outbox leaving 20 messages. After another 60 seconds the final 20 messages would deliver.)