@Rusty @zorinlynx @0xabad1dea Maybe its not a header but something like a client ID or something. Something used for optional authentication that the server just reads its lack or sending a 0 as highly suspicious.Just switching to a different open source mobile mail app solved it on my end, but its definitely an example I can think of where I had an "okay, emails sent from my computer go through, emails sent from my phone are not reaching destination" because it piqued problem solving brain