Hum… I’m running out of ideas, so maybe wait for someone better qualified. I asked a question here SPF: how to cleanly allow thunderbird mail client (#274) · Issues · simple-nixos-mailserver / nixos-mailserver · GitLab but got no answer so far.
You might be able to solve in a dirty way your issue by using ~all or +all instead of -all, but I would be afraid to see the servers considering the emails as spam more easily + would maybe allow anyone to send emails with your name… (maybe enable DKIM & DMARK as specified in the doc to limit this issue?) but in any case might be worth to change temporarily to see what happens.