Web-based outlook app opens *.eml file correctly but desktop-based one hangs

I have an *.eml file that I've received from one of my coworkers. He said that it can't be opened with a desktop outlook app but with a web-based app it opens without any problems.

I checked it and indeed - in the web app it opens like any other message but when I tried to open it in my Outlook for Microsoft 365 app (version 2111 16.0.14701.20254, 64-bit) it hangs and only shows the title and the sender/recipients part.

The file itself is over 2MB in size but I don't think it would be a problem to render it, especially when the browser app is clearly able to do it instantly. It contains a quite long conversation with lots of forwarded messages and some small images.

Unfortunately, I can't show you the message or upload the file for you to check yourself because it's confidential but maybe you had similar issues and you know what's the reason for this weird outlook behavior.

If you want some more info, please ask me in the comment. Thanks

As @LPChip said, if the message body is too long (I don't know the specific limit though), Outlook will not open it.

The "solution" is to open the message file in the text editor, find the body part of the message (usually it begins with Content-Type: text/html or you can just find the largest chunk of encoded data in the file) and delete some of it. If it doesn't work, delete some more, and eventually, it will open.

If you want to decode the message body but you don't have access to a web-based outlook app for some reason just use some online base64-text converter (eg. CyberChef) or use some online *.eml file viewer.