Hi,
In Mail > Preferences > Accounts click on the account in question.
In will show you several tabs (these may vary depending on which version you happen to be using).
This is what I see in a Sierra beta version.

You may have a button suggesting access to all the Outgoing STMP servers.
If so use that and check the tabs there as well and make sure the outgoing server list the correct port, the correct authentication method, and the password if required.
This is an older version Google Mail set up

As you can see my Goolge mail Out-going server was also handling the out going mail for my Lineone account (that was originally Dial-up and then the ISP was taken over by Tiscali and later became Talk Talk)

8:06 pm Thursday; August 25, 2016
iMac 2.5Ghz i5 2011 (El Capitan)
G4/1GhzDual MDD (Leopard 10.5.8)
MacBookPro 2Gb (Snow Leopard 10.6.8)
Mac OS X (10.6.8),
iPhone and an iPad (2)