This is NOT a question, it IS a solution to a problem with Thunderbird mobile. Please read below. (I did not report it as a bug because I'm not sure whether or not the setting is intentional, but it is causing problems.) My web host recently migrated their email servers to #MailChannels. As a result, I suddenly found myself unable to send email outside my domains from Thunderbird #mobile. (However, I could still send to and receive from my own domains, and receive from senders outside my domains -- and Thunderbird desktop was unaffected and working just fine.) After multiple emails were exchanged with web host support, an answer was finally found: in the Thunderbird mobile app, go to General Settings > Privacy and enable the Hide email client option. Doing this removes an incomplete #User-Agent entry from the headers and allows email from Thunderbird mobile to be successfully processed by MailChannels. The web host support technician who relayed this solution to me earlier today also said he had encountered this issue several times today. In my opinion, Mozilla should consider a) removing the incomplete User-Agent entry from the headers; b) enabling the Hide email client option by default; or b) making current and future users of the Thunderbird mobile app of the potential problem and solution so they can determine for themselves the appropriate action. However, it is up to Mozilla to determine the correct course of action, if any. I hope this helps them become aware of the issue so they can make that determination.