Microsoft 365 Email Limitations

This article will be used as a general reference to understand the limitations of Exchange Online, and what checks the Information Technology's Enterprise Architecture team is using before migrating a mailbox to ensure the mailbox does not exceed this limits.

This article will be used as a general reference to understand the limitations of Exchange Online, and what checks the Information Technology's Enterprise Architecture team is using before migrating a mailbox to ensure the mailbox does not exceed this limits.

Exchange Online Limits

The various mailbox limits for Exchange Online are documented here: Microsoft Exchange Online Account Limitations

Pre-Migration Checks

Before starting mailbox migrations to Exchange Online, the Enterprise Architecture team is checking the following stats for all mailboxes:

1. Overall mailbox size
   • To ensure we are finding all mailboxes over the capacity limit, we are searching for all mailboxes greater than 45GB.
2. Single email messages that are too large to move
   • A single message cannot exceed 150MB. To ensure we are finding all messages above the size limit, we are checking for any emails within an inbox greater than 125MB.

If a mailbox is found to be over the 45GB overall size threshold, or if a mailbox contains any messages larger than 125MB, an email is sent to the owner of that mailbox.

1. If the mailbox is over the overall mailbox size limit, the email will ask the mailbox owner to perform general mailbox clean-up (i.e. clear deleted items, deleted any unneeded messages, etc.)
2. If the mailbox contains an email or emails over the 125MB size limit, the email will provide the subject of the email, who it is from, who it is to, and what folder the email is in, and will ask the mailbox owner to delete the email from their mailbox.