It has come up recently, though it has been a issue for quite some time, where people using Outlook may not see the entire email message. I’ve demonstrated this with Outlook 2010, 2013, and 2016. It may be in earlier and newer versions as well.
A situation came about where I replied to a message with instructions on how do do something, the user did not see my portion of the message so things just sat there with the user getting more frustrated. Three weeks later the user inquired again and escalated it as they did not get a response from me previously. With all parties being frustrated I proved that I responded (brought up their copy of the message via web mail rather than outlook) thus diverting some of the frustration onto Microsoft.
I had to dig into the raw message source to find the issue and it turns out to be related to Microsoft Internet Explorer and its use of comments. The original message from the user was sent using Outlook and it was riddled with HTML comments that contained IF statements. When I had originally replied I quoted the relevant section of the user’s message and then added my reply below (top posting is for idiots). Due to the “cut” operation, a comment containing an IF was left without its matching ENDIF. This is what caused Outlook to not display my content to the user.
This situation will show up where multiple email clients are involved (i.e. Outlook vs everyone else) and proper quoting is used (it is “Question then Answer”, not “Answer then Question”).
Now to see if there is a way in Thunderbird to strip all HTML comments from a message.