Last week MailChannels began blocking ALL emails that I send from Thunderbird for Android. Prior to this I had no problems sending from Thunderbird for Android, but MailChannels claim the issue is that Thunderbird is creating malformed headers or message IDs. Other clients for Android are able to send mail without problems. Thunderbird for Windows is able to send mail without problems. Looking at the source of the headers in mail that fails and comparing with mail that is not blocked, the only differences I can see are that Thunderbird for Android is the only client creating messages IDs where the UUID part of the ID is capitalised - other clients use lower-case letters in the UUID. As far as I can tell either is legitimate. The only other difference being the inclusion of the User-Agemt identifying the client as "Thunderbird for Android".. The error message in MailChannels' bounce is as follows: 550 5.7.1 [CS] Message blocked. If this is a false positive, please report this to your hosting service provider. The reason for rejection given on the linked MailChannels page is that the message has been identified as spam content. I have contacted my hosting provider and after a lot of emails exchange (using Thunderbird for Windows!) they have simply said that the problem is with Thunderbird and that I should change my email client - no explanation of what they think is wrong with the headers!