O365 email message limitations

Feature Office 365 Business Essentials Office 365 Business Premium Office 365 Education Office 365 Enterprise E1 Office 365 Enterprise E3 Office 365 Enterprise E5 Office 365 Enterprise K1
Message size limit – Outlook 150 MB1, 2 150 MB1, 2 150 MB1, 2 150 MB1, 2 150 MB1, 2 150 MB1, 2 150 MB1, 2
Message size limit – OWA 112 MB1, 3 112 MB1, 3 112 MB1, 3 112 MB1, 3 112 MB1, 3 112 MB1, 3 112 MB1, 3
Message size limit – Outlook for Mac 150 MB1, 2 150 MB1, 2 150 MB1, 2 150 MB1, 2 150 MB1, 2 150 MB1, 2 150 MB1, 2
Message size limit – migration 150 MB 4 150 MB 4 150 MB 4 150 MB 4 150 MB 4 150 MB 4 150 MB 4
Subject length limit 255 characters 255 characters 255 characters 255 characters 255 characters 255 characters 255 characters
File attachments limit 250 attachments 250 attachments 250 attachments 250 attachments 250 attachments 250 attachments 250 attachments
File attachment size limit – Outlook 150 MB 150 MB 150 MB 150 MB 150 MB 150 MB 150 MB
File attachment size limit – OWA 35 MB 35 MB 35 MB 35 MB 35 MB 35 MB 35 MB
File attachment size limit – Outlook for Mac 150 MB 150 MB 150 MB 150 MB 150 MB 150 MB 150 MB
Multipart message limit 250 parts 250 parts 250 parts 250 parts 250 parts 250 parts 250 parts
Embedded message depth limit 30 embedded messages 30 embedded messages 30 embedded messages 30 embedded messages 30 embedded messages 30 embedded messages 30 embedded messages

1 Office 365 Administrators can specify a custom limit between 1 MB and 150 MB. However, the size of message you can send or receive also depends on what your email client or solution supports. For more information about customizing the maximum allowed message size for your organization, see Office 365 now supports larger email messages.

2 You can send and receive up to 150 MB messages between Office 365 users (where the message never leaves the Office 365 datacenters). Messages that are routed outside of the Office 365 datacenters are subject to an additional 33% translation encoding increase, in which case the maximum message size is 112 MB. 

3 OWA accounts for the possibility that your message may be subject to the 33% encoding increase and restricts the size of message you can send to 25% less than the configured setting. For example, if you customize your settings for a 100 MB maximum message size, you can send messages no larger than 75 MB. 

4 The size of messages to be moved into Exchange Online are computed by Exchange Online. Versions of Exchange prior to Exchange Server 2013 may report a smaller item size. This limit applies to move based migrations using any supported Exchange Mailbox Replication Service. Other migration methods (Cutover, Staged, IMAP, PST) and other third party tools are limited by the general message size limit.

From <https://technet.microsoft.com/en-us/library/exchange-online-limits.aspx#StorageLimits>

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