I don’t know if this is the right place or not…
using Linux-Desktop-1.14 and older
in all versions and across all devices when i try to contact someone new this massage appears:
permanent: 5.7.1  Message rejected under suspicion of SPAM; https://…
and i can’t send massages.
the solution is this:
both you and your new contact should go to your email web page and add each other as contacts and send emails to each other (specially using reply) then somehow the email provider understands that you are friends not spammers.
please tell me if I’m wrong or missing something but this worked for me.