Mailbox Sizes

All mailboxes hosted on Exchange online are capped at 100GB quotas per mailbox (or 50GB if you have an A1 Office365 license).

Message Size Limits

• Email messages have a maximum size that can be sent on our system. Messages must have a total size of less than 150Mb.
• Microsoft Outlook limits sending out email to a maximum of 500 different recipients (a group or list is counted as a single recipient).  If you need to send out more than 500 at a time you can use webmail which has the limits below.
• Exchange Online has a set of mail limits per sender. You can send to: 2,000 external recipients per hour 4,000 internal recipients per hour 6,000 total recipients per 24 hours.
  • If the recipient rate limit is reached, messages can't be sent from the mailbox until the number of recipients that were sent messages in the past 24 hours drops below the limit. For example, a user sends an email message to 3500 recipients at 09:00 AM, then sends another message to 1500 recipients at 10:00 AM, and then sends another message to 1000 recipients at 11:00 AM, hitting the limit of 6,000 messages. The user won't be able to send messages again until 09:00 AM the next day. Messages will be held until the limitation has expired.

Bulk Email

Exchange Online customers who need to send legitimate bulk email  or large mailouts (for example, surveys or customer newsletters) should use third-party providers that specialise in these services. Guidelines for the use of bulk email at Cambridge

Consider using the University of Cambridge mailing list service (Sympa) Getting started with mailing lists or a commercial provider like ConstantContact and Mail Chimp.