Understand content conversion via mail headers and trace logs

[PoorMens_Bravo]

Hi,

We have a situation where some clients revert saying that they are receiving mails in Plain text. The issue is not continuous and intermittent, I have checked the remote domain settings in our tenant and also for any rules, but all of them shows to be good.

However, I do want to understand on how can we look into transport logs & mail header from recipient Outlook & find out where the content conversion is happening.

All of them complaining are external recipients. We are in Hybrid exchange setup, we have Mac Outlook clients as well as Windows Outlook client.

[#2]

@PoorMens_Bravo 

To add, i have checked the domain settings for the remote domain and it says "Follow user settings", i am checking on individual recipients who are added as Guest MEUs and Mail contacts and their UseMapiRichTextFormat, some of them shows to be Never and some of them shows to be UseDefaultSettings

Does that mean for the ones set as Never, will the mail convert to plain text?

[#3]

Have you had a look at this? It covers the locations and settings that may affect whether a message is sent in plain text or not https://learn.microsoft.com/en-us/exchange/mail-flow-best-practices/message-format-and-transmission

[#4]

Hi Dan,
When they receive emails from our end, they see this in their headers
27 Content-Type multipart/related; boundary="_006_DM6P100XXXXXBXXXXXXXXXAXXXXXX519NAMP_"; type="text/plain"

So, I need to find out how it is being delivered as plain text, if that's what the means.

I have checked mail user settings, for UseMAPIRichText, it is being set to UseDefaultSettings
For domain, it ;s follow the user settings.
In Outlook, the default setting sis Rich text.