Message size limit - Outlook
Message size limit - OWA
Message size limit - Outlook for Mac
Message size limit - migration
Subject length limit
File attachments limit
File attachment size limit - Outlook
File attachment size limit - OWA
File attachment size limit - Outlook for Mac
Multipart message limit
Embedded message depth limit
30 embedded messages
First, as a message routes from mail system to mail system it can get larger than when it started. This happens because as a message traverses different mail systems, transcoding (or Base64/MIME conversion) may occur, most commonly between on-premises Exchange and the cloud. This results in a 33 percent larger message than when it left your outbox. Even though the maximum allowed message size is now 150 MB, this is the maximum message size threshold after any potential transcoding has occurred. If your message is going to get transcoded, then it will be limited to 112 MB maximum. Yet, not all messages are subject to transcoding. For example, most messages sent between mailboxes hosted within Office 365 won’t get transcoded, so they can be a full 150 MB in size.
Second, email clients differ in their support for sending large messages. For example, in Outlook, assuming no transcoding is involved, you can attach a 150 MB size file and send it, and assuming the recipient is also hosted in the cloud and is configured to receive 150 MB size messages they’ll receive it. OWA, however, restricts the size of the message you can send to 25 percent less than the configured allowed maximum send size. It does this proactively to account for the possibility there might be a message size increase due to transcoding. For example, if your maximum allowed send size is set to 100 MB then using OWA you can only send up to 75 MB maximum. Additionally, in OWA, each attachment can be no larger than 25 MB. So with a 100 MB maximum send size, using OWA you could attach and send up to three 25 MB files in a single message. These conditions may change in the future, so be sure to check the official Office 365 documentation for the latest on the message size support provided by various email clients.
Lastly, don’t forget that as an Office 365 customer, the current message size limit is 150 MB, you may be able to send and receive large messages up to 150 MB, but your associates may not. Whether it’s because their mail is hosted on a different email service or system that doesn’t support large message sizes, or they’re an Office 365 user but haven’t changed their settings, they may not be able to receive your large message. So if your message is over 25 MB be sure to ask them about it in advance before sending it to them.
Source Office 365 Limit Documentation: https://technet.microsoft.com/en-us/library/exchange-online-limits.aspx