Office 365 email size limit

Exactly how large a message can you have with Office 365?

Office Watch has managed to get to the bottom of the email size mystery with Office 365.

The official size limit for incoming and outgoing messages is stated as 25MB

Behind that number lies a tale.

During the Office 365 beta, the email size limit varied, which was reflected in the different numbers that showed up in the release documentation. At the time we noted numbers between 25 and 35MB in Microsoft’s own web pages and docs.

Now it’s a confirmed ‘25MB’ but notice the quote marks … that’s because it’s not really a hard 25MB limit.

When an email host (like Hotmail, Gmail etc) states a size limit they are usually giving a size for the entire message not just the attachment. The message includes the text, subject, To, From, Date and other technical details as well as any attachment plus some overhead for the attachment encoding. Usually the attachment is the vast majority of the message size and the rest is insignificant. However some people push the size limit and get frustrated when their emails don’t get through. For example, seeing a stated ‘25MB’ limit they send a 25MB attachment and don’t understand why it’s rejected as too large (the message is actually the attachment size of 25MB plus the rest of the message, which is enough to tip the entire package over the size limit).

The Office 365 team understand this, so they announce a 25MB email size limit but the actual enforced limit is a little higher so you can send a 25MB file attachment and it’ll still get through.

‘softies won’t say what the enforced size limit is, but it’s probably in the 26-28MB range so your message will get through unless you have a really long message with a 25MB attachment.

That’s a smart move but Office 365 users should keep in mind that other email receivers might not have the same margin. For example sending a message to a Gmail user, which has a hard 25MB limit, may get refused because it’s too big to receive. Both hosts have ‘25MB’ limits but the actual enforced limits are different.

In practice, it’s better to not push email size limits. Try to keep messages under the stated size limit by a MB or two – that allows for differences in the size definition.

25MB (however it is defined) should be enough for most users. A few JPG images or even a single RAW or TIFF image will rarely get over that limit. Office documents (especially in the compressed Office 2007/2010 formats) are usually well under that limit too.

We have an article on how can you email a file that’s too big to be sent while Office 365 users can always use Sharepoint to share larger files with others on the same system.


Another Send limit

Another Office 365 limit is on the number of recipients.  Each user is limited to 1,500 email recipients per day.

This is to prevent Office 365 being used by spammers and should be more than most users need.  However it might be a limitation if you send Word ‘mail merge’ emails to a large number of customers.