Receiving email with OpenSMTPD fails with 550 Invalid recipient

I am trying to set up a mailserver using OpenSMTPD. My configuration is:

listen on enp4s0f2
action "save" maildir "/mail/%{rcpt}"
match for rcpt-to regex ".*" action "save"

/mail is globally rwx. journalctl logs for when an email arrives:

Apr 28 08:06:31 greysky smtpd: info: OpenSMTPD 7.5.0-portable starting
Apr 28 08:07:00 greysky smtpd: a99185e682bda8d6 smtp connected address=209.85.218.43 host=mail-ej1-f43.google.com
Apr 28 08:07:01 greysky smtpd: a99185e682bda8d6 smtp failed-command command="RCPT TO:" result="550 Invalid recipient: "
Apr 28 08:07:01 greysky smtpd: a99185e682bda8d6 smtp disconnected reason=quit

where domain.com is the server's domain. I am using NixOS, in case that is relevant.