[mark700]

have you tried the protonmail provided @pm.me extension? This has worked for me for getting past spam filters. the @protonmail.com extension almost always gets forwarded to spam. if you dig around in the settings of your email, you should find this option under Account Settings -> Addresses -> Default. i can't recall, but it may be part of the paid version of protonmail.

[WithinReason]

That's probably because you have to pay to send using pm.me, and spammers use free accounts