Use Server Settings preferences in Mail to change options for an account’s incoming and outgoing (SMTP) mail servers.
To change these preferences in the Mail app on your Mac, choose Mail > Preferences, click Accounts, then click Server Settings.
Note: The settings shown vary depending on your account type.
Option
Description
User Name
Enter the user name you use on the mail server.
Password
Enter the password you use on the mail server.
Host Name
Enter the name of the mail server.(mail50XX.site4now.net, you can get it from Control Panel>>Emails>>Manager Emails user panel)
Automatically manage connection settings
Select Use TLS/SSL.
Port (993 for IMAP or 995 for POP3),
Authentication: The authentication method. This option is available only for POP and IMAP accounts.
Account
Choose the account of the mail server.
To change outgoing server settings, as directed by your email account provider, choose Edit SMTP Server List.
Enter the name of the mail server(mail50XX.site4now.net, you can get it from Control Panel>>Emails>>Manager Emails user panel)
Port (465 or 587).
If you change settings, click Save to save your changes.
For your reference please check this screenshot: