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)

And

Maximum size for received messages (KB)

7. 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.