Increase or decrease the maximum attachment size

Asset 25

Change the maximum message size for sent and received messages in O365

Posted on 20 February 2019 by Beaming Support

This guide explains how Office 365 administrators can increase or decrease the maximum attachment file size allowed on emails both sent and received.

If you’ve been receiving lots of attachments that are not legitimate or contain sales documents you are not interested in, then you may wish to slightly reduce the maximum size for received messages to stop the spam from reaching you.

Alternatively, you may have multiple or large documents that need to be sent but you’re unable to send them via email due to attachment size restrictions.

Within Office 365 Exchange Admin Center an administrator is able to set and amend message size restrictions as follows:

  1. Log into Office 365 as an administrator
  2. Navigate to Exchange Admin Center
  3. Open Recipients and ensure the ‘Mailboxes’ option is chosen.
  4. When a mailbox is selected choose the symbol at the top of the page that looks like 3 dots ‘…’
  5. ‘Then choose set default mail size restrictions’.
  6. From this menu you will see:

Maximum size for sent messages (KB)


Maximum size for received messages (KB)

  1. Set your preferred mail sizes.

If the maximum size for sent messages is increased this will then allow the user to send emails with larger file sizes. By decreasing the max file size for received messages, any emails that are over that size will not reach your mailbox and will be rejected. This helps to prevent companies sending you sales emails with multiple catalogues etc and helps to prevent your mailbox from reaching its maximum size, which would otherwise stop standard emails from reaching you.

How long should my file take to transfer? Use our calculator to find out.

Sending a lot of files?

Most internet connections have an upload speed much slower than the download speed. If you transfer and upload a lot of large files you may want to look at a connection with synchronous speeds.

Top tips direct to your inbox

As an Internet Service Provider for businesses, we’re full of good advice.

Our monthly email updates bring you cyber security guidance, tips to help you get the best out of your connectivity and lots of how-to guides for the services you use online, like Office 365.

  • This field is for validation purposes and should be left unchanged.

Business cyber security advice.

Straight to your inbox every month.

  • How-to guides to boost productivity and efficiency
  • Changes in voice and internet tech that your business should know
  • Cyber security advice anyone can follow

  • This field is for validation purposes and should be left unchanged.