Yes interesting sentence. This need some extra words I assume? CC @wxiaoguang https://github.com/go-gitea/gitea/commit/466d6ba3f576ab8b04aa33f6eee488c1131de9f5

```suggestion ;; you can set SMTP_PORT instead and this will be inferred. ;; Since 1.18 ;PROTOCOL = ```

Maybe we need break notice here.

Should the install form be updated accordingly? https://github.com/clarfonthey/gitea/blob/mailer-settings/templates/install.tmpl

The logic should be if `SMTPAddr` is not set but `HOST` set, then fallback.

The logic should be if `SMTPAddr` is not set but `HOST` set, then fallback.

Old setting should be kept some versions and marked as deprecated.

Old setting should be kept some versions and marked as deprecated.

This error doesn't sound user-friendly.

Could we use [`net.(IP).IsLoopback()`](https://pkg.go.dev/net#IP.IsLoopback) here?

Generally L-G-T-M. I prefer to use a URL to set all these protocol/addr/port. eg: ``` SMTP_ADDR = smtp+starttls://username:password@smtp.gmail.com:587 ``` And IIRC there are some SMTP options on the installation page. Personally I prefer to remove these options from the installation page, but I don't know how others think about it .....

```suggestion host := "localhost" if opts.Protocol == "smtp+unix" { host = opts.SmtpAddr } ```

```suggestion return fmt.Errorf("could not load provided client certificates: %v", err) ```

A NPE can happen here if a user does have client cert specified but not the correct protocol(should probably be warned about in setting/mailer.go)

`HELO` is now by default disabled? ```suggestion EnableHelo: sec.Key("ENABLE_HELO").MustBool(true), ```

> If you don't know what this is, leave it blank. I think this should be left out. See relevant discussion: https://github.com/go-gitea/gitea/pull/17440#discussion_r752890770 Also could the default value be added for this key?

Multi line comments are preferred 😅