SMTP host or IP. When using SMTP it is recommended you use a stable SMTP service, like SparkPost or MailGun for mail delivery. DO NOT use your own server settings. Once your email details are in place use the test button to send a test message. If you still have issues, enable the mail debug log. Info below.
IMPORTANT: If you are using Gmail, DO NOT use your Gmail password. You MUST create an app specific password in your Google account and use that. Also, ONLY use SSL if you have SSL installed. If you don`t, you should use TLS. For the Gmail port ONLY use 465 or 587.