Having set the Filter content to Yes - regardless of other setting in the "Alter Messages" tab, make a HTML message come out empty. I can replicate this on my test list.
I have tried debugging by changing each setting one-by-one and sending the user's mail, but no change makes the mail come out with a content. However setting "Filter content" to "no" makes the mail come out okay.
With "Filter content" set to "Yes", sending another HTML mail is okay, so something in the user's mail may not be right, but I wonder why setting "Filter content" to no "solves" the problem. Afte all this setting is switching all other settings on and off, so I can't just ask the user to turn it off.
May 06 11:53:32 2019 (33) ACCEPT: <250E794B36C1E646AF3D81E1F15348C79DA8CA83@P2KITMBX06WC03.unicph.domain> May 06 11:53:32 2019 (37) [LISTNAME]@lists.example.com invalid FilterAction: discard. Treating as discard May 06 11:53:32 2019 (37) <250E794B36C1E646AF3D81E1F15348C79DA8CA83@P2KITMBX06WC03.unicph.domain> discarded by "default-posting-pipeline" pipeline handler "mime-delete": After content filtering, the message was empty May 06 17:27:35 2019 (33) ACCEPT: <250E794B36C1E646AF3D81E1F15348C79DA8CAEC@P2KITMBX06WC03.unicph.domain> May 06 17:27:35 2019 (37) [LISTNAME]@lists.example.com invalid FilterAction: discard. Treating as discard May 06 17:27:35 2019 (37) <250E794B36C1E646AF3D81E1F15348C79DA8CAEC@P2KITMBX06WC03.unicph.domain> discarded by "default-posting-pipeline" pipeline handler "mime-delete": After content filtering, the message was empty
/Henrik Rasmussen