Might need an SMTP session log (with an actual SMTP error code for each attempt) to troubleshoot that further.
I just did a basic check of Purism’s SMTP server and it seems to be OK but I didn’t send an actual email. (Check covered: MX record lookup, corresponding A record lookup, TCP connect, STARTTLS succeeded - hence certificate OK, my sender address accepted, the above recipient address accepted.)
Can you send from something other than Yahoo?