I have a lots of emails stored in local folders but also some IMAP accounts configured in Thunderbird. OS: latest Ubuntu 24.10 Thunderbird: 128.9.1esr (64 bit) from snap Recently I tried to move some emails from local mbox to Proton Mail (via their Proton Mail Bridge) running on the laptop. But I wasn't able to move the email. After some discussion with Proton support, it turns out that there is incompliant headers in the emails (see one of those emails full headers below). The X-Mozilla-Keys row is full of spaces, folllowed by newline character (^M). The following line start with ">>>>>>" and this is the reason why Proton Mail Bridge refuses to move this email to Proton account. What is the problem? Thunderbird never complained about the email. (EDIT: Headers removed.)