Recently we came across an issue on an Exchange 2010 environment where all mails sent via OWA and Outlook 2011 (MAC) were getting received as Plain Text mails externally even though we specifically had set OWA to send in HTML.
Internally Users could send HTML messages via OWA and they would get received by the users as HTML as we wanted. The first thing we did was to compare what transport rules were getting applied to external mail and not internal mail and visa versa. As a test we got a user to have the same transport rules that were getting applied when sending internally but apply this to any external address also, however the issue still remained so we had ruled that out.
The second thing we looked at was the send connectors, as they were set to send mail via Exchange Hosted Services so we wanted to rule out that anything was going wrong at this end. So we basically created a new send connector which would basically route mail via DNS MX records to a specific address space (i.e hotmail.com , domain.com) therefore bypassing EHS. We then tested again but the issues still remained.
The final thing we checked which revealed what the issue was to run the following exchange powershell “get-remotedomain | fl” . We then spotted that the content type was set to MimeText which “converts all messages to MIME messages that use text formatting” (i.e plain text). We then changed the content type to mimehtmltext (Converts messages to MIME messages that use HTML formatting, unless the original message is a text message. If the original message is a text message, the outbound message is a MIME message that uses text formatting) by running the following command “get-remotedomain | set-remotedomain -ContentType MimeHtmlText”. Please Note You will have to alter the previous command if you have multiple remote domains so that you change the content type on the specific remote domain you want.
After the above change everything worked perfectly 
Thank you, this resolved the same issue for us.
We are running Exchange 2007 SP3 and were having issues with all external mail from Mac Outlook 2011 clients being converted to plaintext.
Thanks, this worked for us as well!
We went from Exchange 2003 to 2010 and upgraded our Mac clients to Outlook 2011 and the exact issues as described were occuring. Running the powershell command fixed the issue right away.
Thank for this. Had exactly the same issue and it’s an instant fix.
I’ve added this tip to my blog and linked back to you, thanks.
Absolutely genius! Running the powershell command fixed the problem immediately.
Thanx
Hi, I’m experiencing exactly the same problem! Although I’m not quite familiar enough with Exchange to fully understand the proposed solution here. A bit of Googling on Exchange powershell got me not a lot further. Am I (as a user) able to solve this problem myself? Or can this only be solved by the administrators of the Exchange server(s)? I fear the latter is the case.. If I can solve this myself, how can I do that?
Any help is greatly appreciated!
Kind regards,
Pieter
Thanks, this corrected our issue as well!!!
Your Exchange Admin would need to do this change
@Daniel Davies
Feared so.. thanks for the info though.
Thanks so much for the information Daniel, really deeply appreciated there.
Awesome! I’ve had exactly the same issue with Exchange 2010 SP1.
Your help is also greatly appreciated.
Great info! Worked perfectly.
Thanks for posting, and for the detail on both the problem and fix.
Much appreciated.
Thanks for the solution. My customer are very Happy
Best regards
Julien
Just to add there is a work around to force iOS devices like the iPhone and iPad to also send email in HTML format by doing the following on the iOS device:
iPhone Settings > Mail > Signature
Option 1 – Add HTML formatting to the existing text “Sent From My iPhone”.
Select the text (Select All, or some of the text)
On the menu that appears, press the > (right arrow), and select the BiU (bold/italics/underline) button. Select one (or more) of those and now HTMl formatting will work.
Option 2 – If you do not want “Sent From My iPhone”
Insert a full stop/period. Highlight that and add formatting to it via the method described in Option 1.