when someone receives an email than goes to View Source in case the email has attachment few MB, the browser will stuck as it tries to open the whole source.

I'd suggest to fix it by opening the source without the attachment.

I did try to open View Source email with 3MB attachment on FF 35.0.1 on Windows 8.1.

We're using
UI version: 7.6.1 Rev12
Server version: 7.6.1-Rev16