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Exchange Email Message Throttling and Limits
Article: KB0010838 Published: 09-13-2017 Last modified: 09-13-2017 Views: 66
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As part of the expanded anti-spam/blocklist prevention measures, Virginia Tech uses sending rate limits on Exchange email services. Individual Exchange email accounts are limited to a message rate of 30 emails per minute and a unique recipient rate limit of 10,000 recipients per day.

Recipient Rate Limit

10,000 per day

This is the maximum number of unique people sent to in a 24 hour period. Exceeding this limit causes any messages above the limit to be rejected with a bounceback NDR until the next 24 hour time period starts.

Message Rate Limit

30 per minute

This is the maximum number of email messages the server will send out within 1 minute. Excess messages are queued up within Exchange's internal mail queues and sent out at this rate limit. No rejections or bouncebacks will appear, but deliveries will be delayed if over the limit as the queue empties.

What Counts as Recipients and Messages?

If the recipient is an object that Exchange itself is aware of as a singular object and does the breaking out for the users, it counts as one object not as each individual member.

For example, sending a message to an Exchange distribution list 'bigroup-resc' of 500 people would count as just one recipient and one message. An external group or list hosted by something other than Virginia Tech is the same, because Exchange cannot see the number of members in an external group or list in order to count them. If you send to 'biggroup-g' Google group of 500 members, Exchange only sees and counts that single 'biggroup-g' email address as one recipient and one message.

The only place that does count individual members is from an Outlook Contact Group. Because those objects aren't really a single object; they are a folder where the Contact Group creator placed multiple recipients, a grouping feature that Outlook offers. If you send to a Contact Group containing five people, that counts as five recipients. In Outlook when you send to a Contact Group it populates the To field with each individual member; the Group itself doesn't really exist anywhere.

In a case where both distribution lists or Google groups and individual people are present in an Outlook Contact Group, the Group will break out what it can but a distribution list is still a single object. A message to a contact Group made up of five distribution lists with 500 members each would count as five recipients instead of one or instead of 2500, because the Contact Group gets expanded but cannot expand the distribution list.

CC (carbon copy) and BCC (blind carbon copy) are treated normally like any other To: field. For example, if I send a message to Bob and CC Frank and BCC Tom, that counts as three unique recipients and one message.

Examples:

  • Sending a message to a distribution list containing 500 members = 1 recipient, 1 message
  • Sending a message to a Google group of 500 = 1 recipient, 1 message
  • Sending a message to an Outlook Contact Group containing 500 contacts = 500 recipients, 1 message
  • Sending a message to an Outlook Contact Group containing five distribution lists/Google groups with 500 members each = 5 recipients, 1 message
  • Sending a message to Bob, CC'ing Frank, BCC'ing Tom = 3 recipients, 1 message

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