UTF-7 is not just a subset of UTF-8. To support it, a real decoder is necessary.

https://datatracker.ietf.org/doc/html/rfc2152

My mistake, sorry.
Since go-imap supports utf-7, can we use same code in go-message?
https://github.com/emersion/go-imap/tree/master/utf7

go-imap supports a modified version of UTF-7, defined by the IMAP RFC. Maybe it can be adapted.

But I'd rather not add it built-in to go-message, I'd rather let users manually set CharsetReader to a function which handles UTF-7 if so they wish.

Disagree.

Since utf-7 is not used by only one language (e.g. Chinese, Korean, Japanese), it's better make it a built-in support to benefit all go-message users. :)

go-message has no built-in charset support apart from utf-8 and ascii. utf-7 is no different than say gbk or shift-jis.

Thanks for the explanation.

Just in case if somebody will need it:

package charset

import (
	"github.com/cention-sany/utf7"
	"github.com/emersion/go-message/charset"
)

func init() {
	charset.RegisterEncoding("utf7", utf7.UTF7)
	charset.RegisterEncoding("utf-7", utf7.UTF7)
}

then import it instead of _ "github.com/emersion/go-message/charset":

 import (
-	_ "github.com/emersion/go-message/charset"
+	_ ".../charset"
 )