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