The certificate for this server is invalid

I just set get a new job with a POP email account.

I can add the account to gmail, and my android phone, but I cannot add the account to Apple Mail.

I am operating a macbook pro with macOS High Sierra.

When I add the count I get the error "The certificate for this server is invalid." I can click "ok" or a ? Ok just shows the account is offline. Clicking "?" pops up a window that says:

The selected content is currently unavailable

The content you selected can’t be displayed at this time. Try again later.

You may be experiencing one of the following issues:

• • • The server may be unreachable.
    • Your school, business, or Internet service provider (ISP) may be experiencing an outage.
    • The content may not exist.

In the middle of writing this I tried to click the lightning icon showing the account was offline and the icons disappeared. I tested mail to see if I could send a message, and when I clicked send I got this popup:

Verify that you have addressed this message correctly.

Check your SMTP server settings in Mail preferences and verify any advanced settings with your system administrator.

The server response was: <m85-94-174-119.andorpac.ad[85.94.174.119]>: Client host rejected: Access denied

Select a different outgoing mail server from the list below or click Try Later to leave the message in your Outbox until it can be sent.

Sending from: "my address>

When I try to change the advanced account settings, and edit the server or SSL info, my application doesn't have the same options as some people suggest it should. I keep getting told it cannot verify my username and password, but I know they are correct because I can log in on the phone and through gmail.

I followed the instructions here, with no luck:

https://discussions.apple.com/thread/8355582?answerId=33281140022#33281140022

Please help.