Transcoding Exchange Online Message sizes

With regards to the recent announced support for e-mail messages up to 150 MB it is important to note that the new limits only applies to non-transcoded messages that is handled in the Office Outlook client.

When a message is routed between mail systems, overhead may be added due to transcoding, or Base64/MIME conversion. Arriving at the destination, the message may be up to 33% larger compared to when it was sent.
Even though 150 MB is the maximum message limit, this threshold is only for the final message – that is after any transcoding has been added. If a message is trancoded, the maximum message limit will then be 150/1,33 = 112 MB.

But not all e-mail messages are transcoded though. Messages sent using Office Outlook from one Office 365 mailbox to another Office 365 mailbox will not get transcoded, an will get full limit of 150MB.

When using Outlook Web Access (OWA), any messages are limited in advance by 25% of the maximum of 150MB to pre-adjust for any downstream transcoding, so the final message size is limited to 112MB.

The full message size limit of 150MB also applies during mailbox migrations when using the Exchange Mailbox Replication Service (MRS).

When using Exchange Web Services during third-party migrations, the maximum message migration size is limited to 64MB with a final message size of approx. 48MB, due to overhead of 33%.

Lastly, the maximum message size for Mac Outlook clients has not changed from the limit of 35MB for Send, 36MB for Receive and 25MB as the published default, after transcoding.

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