Incoming and outgoing mail are handled a bit differently from one another in Mail.app. This is helpful as you can have multiple outgoing accounts even if you only have one incoming accounts, useful if you move between a lot of different networks.
To setup a new SMTP account, open the Accounts pane of the Mail.app preferences and choose Edit Server List… from the Outgoing Mail Server (SMTP): field. At the Server List screen click on the “+” icon and enter a description for the account. This can be anything to help the end user remember what this account is meant for. Then type in the name or IP address of the server in the Server Name: field. You can click on the Advanced tab here to customize the port, enable SSL or set the authentication type/parameters. Once you are satisfied with your settings, click on the OK button or click on the “+” sign to create another outgoing mail account.
Having multiple SMTP accounts can be confusing though. To remove an existing SMTP account, open the Accounts pane of the Mail.app preferences and choose Edit Server List… from the Outgoing Mail Server (SMTP): field. At the Server List screen click on the “-” icon while highlighting the account you would like to remove.
