Office 365 message limits

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Office 365 has some unusual limits on the number and size of messages you can send or receive.

Office 365 has some restrictions on emails and how many you can send, but some are not calculated the usual way.


Message Size

The size of an email message is always calculated ‘all up’. Not just the message and attachments are included. The message header and any ‘overhead’ from file attachment is also in the total size.

The official message size limit (incoming or outgoing) is 25MB. Little birds at Microsoft tell us that the set limit on the Office 365 servers is a little higher than that – maybe 30MB or even 35MB (depending on the bird). Redmond did that to prevent hassles with messages that are near the published 25MB limit. It’s a nice touch, but it’s best that you keep the 25MB limit in mind rather than pushing it.

The other limits are probably beyond any normal email use and not worth worrying about. There’s a maximum of 125 attachments per message and 30 embedded messages. Subject lines have to be less than 255 characters – much less in practice.

Outlook Web Apps – you are limited to 10MB for each individual attachment to an outgoing message (ie uploaded via the browser).


Sender limits

Like any email hosting system, there are limits on the number of outgoing messages. This is to prevent an account being used for spamming. The trick for providers is to set a limit which is more than enough for customers but too low to be of appeal to spammers.

Normally there’s a global limit on the number of messages that can be sent per day, week or month. Office 365 takes a different approach by limiting the number of recipients per day.

There’s a limit of 500 message recipients per user per 24 hours (1,500 for enterprise plans). A recipient is a single entry in TO CC or BCC fields.

The trick is that a distribution list is counted as a single recipient – even if the distribution list contains hundreds of recipients.

If you breach the 500 recipients limit you can’t send any more messages. You have to wait until the count for the last 24 hours drops below the magic 500 number.

The bottom line is that Office 365 isn’t suitable for broad emailing to customers.


Message Sending rate

There is a limit of 30 outgoing messages per minute per Office 365 user.

You might hit this limit in a few, not so unusual, circumstances:


Reconnection after a period offline

If you’ve been offline and busy working your Outbox might have over 30 messages. When Outlook reconnects to Office 365 servers it’ll try to send those messages as soon as it can so it’s possible that the ’30 per minute’ limit will be breached (depending on the size of the emails and speed of connection).

It should not be a problem, Office 365 will refuse to accept the ‘over limit’ messages from your Outlook. But Outlook will try again in a minute or two and the messages will go.


Mail Merge

Doing a mail merge (probably using Word) will generate a lot of messages quickly and could well exceed the per minute sending limit.

Again, Outlook should cope by holding the messages that Office 365 won’t accept and resend them a minute or two later.

With a mail merge, make sure you don’t exceed the 500 recipients per 24 hours.

 

There are other Office 365 message limits but most of them are so high that they should not trouble most users. They are detailed here and here.

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