This appears rather easy to reproduce. Just include the char 0x0c in a message, and the signature will be invalid. Playing with the resultant message, I can make it valid by removing the \x0c character, suggesting that it's being stripped from whatever rspamd receives for signing.
- ^L causing DKIM validation errors with opensmtpd+rspamd Jason A. Donenfeld
- Re: ^L causing DKIM validation errors with opensmt... Jason A. Donenfeld
- Re: ^L causing DKIM validation errors with ope... Jason A. Donenfeld
- Re: ^L causing DKIM validation errors with... Demi Marie Obenour
- Re: ^L causing DKIM validation errors ... Jason A. Donenfeld
- Re: ^L causing DKIM validation er... Jason A. Donenfeld
- Re: ^L causing DKIM validatio... Jason A. Donenfeld
- Re: ^L causing DKIM valid... Jason A. Donenfeld
- Re: ^L causing DKIM valid... Jason A. Donenfeld