Add quoted-printable decoding to smtp server

Some e-mails are sent using quoted-printable encoding [0], resulting in
notifications with weird characters.

This commit adds support for this encoding, resulting in the following:

**Before**
```
A
=3D=3D=3D=3D=3D
B
=3D=3D=3D=3D=3D
C
```

**After**
```
A
=====
B
=====
C
```

[0] https://www.rfc-editor.org/rfc/rfc2045.html

server/smtp_server.go

@@ -9,6 +9,7 @@ import (
 	"io"
 	"mime"
 	"mime/multipart"
+	"mime/quotedprintable"
 	"net"
 	"net/http"
 	"net/http/httptest"
@@ -265,6 +266,8 @@ func readMultipartMailBody(body io.Reader, params map[string]string, depth int)
 func readPlainTextMailBody(reader io.Reader, transferEncoding string) (string, error) {
 	if strings.ToLower(transferEncoding) == "base64" {
 		reader = base64.NewDecoder(base64.StdEncoding, reader)
+	} else if strings.ToLower(transferEncoding) == "quoted-printable" {
+		reader = quotedprintable.NewReader(reader)
 	}
 	body, err := io.ReadAll(reader)
 	if err != nil {