Message size limit for HVE

Question

I am looking at enabling High Volume Email (HVE) for some applications to send SMTP messages. I need to know what the message size restrictions for this are. I have seen in 3rd party blog posts they are saying there is a 10mb attachment size limit, but I don't see this mentioned in any official Microsoft documentation for HVE, so I am trying to find the correct answer.

Thanks

Answer 1

Hello, @Geezer32,

Welcome to the Microsoft Q&A platform!

After carefully reviewing Microsoft's official documentation on High Volume Emails, it is true that no information on message size limits can be found in the existing documentation.

But same, I found the answer you see in several third-party documents about HVE, as shown below:

（Screenshot from: https://www.powershellcenter.com/2025/01/05/hve-guide/.）

（Screenshot from: https://practical365.com/exchange-online-hve/.）

(Notice: Microsoft provides third-party contact information to help you find additional information about this topic. This contact information may change without notice. Microsoft does not guarantee the accuracy of third-party contact information.)

After summarizing these third-party articles and the discussion below, 10Mb seems to be the more accepted answer at this point. I will update if Microsoft comes out with documentation on this later.

Should you need more help on this, you can feel free to post back. 

---

If the answer is helpful, please click on ACCEPT ANSWER as it could help other members of the Microsoft Q&A community who have similar questions and are looking for solutions.

Thank you for your support and understanding.

Best Wishes,

Alex Zhang