I run my own email server and never have any trouble with email delivery – however recently some steam emails failed to arrive.

I checked I wasn’t in any blacklists with senderbase, then dug further.

Turns out I had SSL enabled on my MTA but using my old cert which was SHA1 & revoked (with a SHA-256 replacement issued), replacing that fixed mail deliveries.

Normally MTAs don’t care about the validity of MTA certs (a lot of them are self-signed) so I hadn’t bothered replacing the cert there – only on website, but it seems Steam does.