Also, if you run your own sendmail you can just add the following completely readable snippet to your sendmail.mc: LOCAL_CONFIG Kplus regex -d+ -s1,2 ^([^.

Also, if you run your own sendmail you can just add the following completely readable snippet to your sendmail.mc: LOCAL_CONFIG Kplus regex -d+ -s1,2 ^([^._]+)[._](.+)$ LOCAL_RULE_3 R$* <@ $=w > $* $: $(plus $1 $) <@$2> $3 That allows you to use "username.whatever@example.com" or "username_whatever@example.com" as well as the "+". A lot of web sites' idiot email validator regexes won't allow "+" in emails. But every single one allows "." and "_".