• The goal is simple. SMTP plugin will use a valid mail adress to send notifications.
• Install the plugin (from extension manager) and go to Configuration settings > Smtp section.
• To configure it, you need your outgoing server parameters provided by your mail server.
• For example, Your outgoing SMTP server. might be pro3.mail.ovh.net
• The port your SMTP server listens on. Usually 25. 465 for SSL. might be 587
• What kind of encryption is used when communicating with your SMTP Server? might be TLS
• If authentication is required, put your user name here. has to be your valid mail adress
• Password for the above user. the password that you normally use to auth in your mail adress.
• In configuration settings, notification section, check if you have a valid mail adress to recieve notification (notify) AND the same mail that is filled in SMTP configuration for mailform.
• You also need to check in dokuwiki.php file if $conf['registernotify'] and $conf['mailfrom'] are well filled.

Once again all these parameters are specific of your mail provider. If you struggle to find this parameters, you can search how to configure mail forwarding <your mail provider> or Set up your email address on the Gmail online interface and find outgoing server server parameters. Indeed, you need the same parameters that are required to send email from another mailbox that your mail provider.

Source(s) / Useful link(s) : https://www.dokuwiki.org/plugin:smtp