Merge branch 'ag/doc-send-email'

The `send-email` documentation has been updated with OAuth2.0
related examples.

* ag/doc-send-email:
  docs: add credential helper for outlook and gmail in OAuth list of helpers
  docs: improve send-email documentation
  send-mail: improve checks for valid_fqdn

git-send-email.perl

@@ -1359,7 +1359,9 @@ sub process_address_list {
 
 sub valid_fqdn {
 	my $domain = shift;
-	return defined $domain && !($^O eq 'darwin' && $domain =~ /\.local$/) && $domain =~ /\./;
+	my $subdomain = '(?!-)[A-Za-z0-9-]{1,63}(?<!-)';
+	return defined $domain && !($^O eq 'darwin' && $domain =~ /\.local$/)
+		&& $domain  =~ /^$subdomain(?:\.$subdomain)*$/;
 }
 
 sub maildomain_net {