I just wanted to share a problem I had with external email warning and plain text (HTML no issues).
For me, the warnings by default were showing only in the headers and not in the body of the email so my users could not see them.
The solution was simply to make line 1 in the inline.external.warning.txt blank and start the warnings on line 2.
It wasted some time for me and I am hoping maybe it could save someone else time who may have this issue down the line.