Enable easy setup for Email pop/smtp via one host

I have directadmin on a dedicated server, and have outlook setup for the account as follows:

both incoming and outgoing: mail.eachclientsdomain.co.nz

my outgoing server smtp requires authentication
- same settings as incoming

advanced tab - outgoing port 587, and tls encryption

But outlook reports the following:

The server you are connected to is using a security certificate that cannot be verified. The target principal name is incorrect
view certificate

Do you want to continue using this server?


The certificate shows - This CA Root certificate is not trusted. To enable trust, install this certificate in the trusted root certification authorities store
Issued to: localhost
issued by: localhost
valid from 3/9/2009 to 18/1/2037 - ie from when my dedicated server was installed.

While I can certainly continue to click "Yes" to trust the cert, I am not keen on this continuing for my clients.

How do I get a certificate that can be trusted in such a way that this error does not continue.

I would like it working not for just mydomain.co.nz, but for each of my clients domains.

I have seen comment that tls is the new ssl, and I have seen port 587 recommended. I can send emails fine, so its working. I just need to make whatever changes are required so this error message does not show.

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