Mailbox size & message limits

Mailbox size

Office365 mailboxes have a large storage capacity of 50Gb.  When you get near to this limit you will get warning messages.

It is good practice to keep your mailbox tidy - see the guide at Mailbox cleanup


Message limits

The maximum size of an individual message that you can send or receive in O365 is 25Mb, including any and all attachments.

If you try to send an email with attachment(s) greater than 25Mb you will get an error message as shown below :

pop-up error message for breaching O365 Email message size limits






Message size limits are necessary to prevent large messages from blocking delivery of other messages and affecting service performance for all users. These limits include attachments, and apply organization-wide to all messages (inbound, outbound, and internal). Messages larger than this limit will not be delivered, and the sender will receive a non-delivery report (NDR).

For full details of O365 email limits please see the Microsoft Technet page for Exchange Online Limits


Mailing Lists

The maximum size of a message you can send to a University mailing list is 2.5Mb.

Mailing lists designed for all students and staff have a limit of 64k.  


How can I reduce the size of my message?

  • Rather than adding large attachments to an email, consider putting the content on a shared storage space or drive and linking to it in your email
  • If you want to check the size of a message before sending, save it to Drafts, then open the Drafts folder where you can check the size of your message
  • Signatures containing pictures and other formating can add considerably to the size of a message. When sending to a mailing list, consider using a more simplistic signature