Customer using a VPN, Apple mail client won't send mail

I have a customer with an iMac pc
He uses a VPN…
He has trouble sending mail from his Apple mail app through my virtualmin postfix mail server (I am using ssl certs on server)
He seems to be able to receive but not send. He was able to send mail the other day, but suddenly now he cannot.

When I configure our own iMac with same settings as his Apple mail client, I have no problems sending mail through his domain email account. Using either START TLS or just SSL (so I can run ports 143/587 and 465/993 no problems)

What is likely his issue? Could it be his VPN? Can a letsencrypt ssl renewal cause this?
What would look for in server logs for problem solving this and which logs?

Maybe ask him for his VPN login details and try to use that on your iMac and see what happens?

Possibly VPN blocking mail ports to stop spammers?