User mailbox storage limit The maximum amount of all email which can be stored in the users mailbox (including attachments). This is a vendor limit set by Microsoft and mailbox storage cannot be increased past this size.

 100 GB

Shared mailbox storage limit The maximum amount of all email which can be stored in the shared mailbox (including attachments) 

 50 GB

Prohibit send capacity alert The user receives a prohibit-send notification email when the mailbox size limit is reached. The user cannot send new messages until enough email is deleted to bring the mailbox below the size limit.

 49.5 GB

Message size limit The maximum total size of an email message. The total size includes the message header, the message body and any file attachments.

 25 MB

File attachments limit The maximum number of file attachments allowed in an email message. Even if the total size of all the file attachments doesn't violate the message size limit, there is still a limit on how many attachments are allowed in a message.

 250 attachments

Subject length limit The maximum number of text characters allowed in the subject line of an email message.

 255 characters

Recipient rate limit To discourage the delivery of unsolicited bulk messages, Exchange Online has recipient limits that prevent users and applications from sending large volumes of email. These limits apply to all outbound and internal messages.

 10,000 recipients a day

Recipient limit The maximum number of message recipients allowed in the To:, Cc: and Bcc: fields. A group is counted as a single recipient.

If you are using Office 365 you will not be able to send an email to more than 100 recipients in a single email, but can be increased by contacting the IT Service Desk.

 100 recipients

Message rate limit The maximum number of e-mail messages that can be sent from a single e-mail client per minute. The client is identified by the user account.

 30 messages per minute

Deleted Items folder retention period The maximum number of days that items can remain in the Deleted Items folder before they are automatically removed.

 30 days

Retention period for items removed from the Deleted Items folder The maximum number of days that items removed from the Deleted Items folder are retained before they are permanently deleted.

 14 days

Junk Email folder retention period The maximum number of days that items can remain in the Junk Email folder before they are automatically removed.

 30 days