Add hint how to urlencode special characters in EMAIL_CONFIG variable of .env file

deploy/env.prod.sample

@@ -52,6 +52,12 @@ LOGLEVEL=error
 # EMAIL_CONFIG=consolemail://         # output e-mails to console (the default)
 # EMAIL_CONFIG=dummymail://          # disable e-mail sending completely
 # On a production instance, you'll usually want to use an external SMTP server:
+# If `user` or `password` contain special characters (eg.
+# `noreply@youremail.host` as `user`), be sure to urlencode them, using
+# for example the command:
+# `python3 -c 'import urllib.parse; print(urllib.parse.quote_plus
+# ("noreply@youremail.host"))'`
+# (returns `noreply%40youremail.host`)
 # EMAIL_CONFIG=smtp://user@:password@youremail.host:25
 # EMAIL_CONFIG=smtp+ssl://user@:password@youremail.host:465
 # EMAIL_CONFIG=smtp+tls://user@:password@youremail.host:587