1. Sub-domain addressing for users with _ (underscore) in their name by using - (hyphen) instead

Sub-domain addressing is a feature where if you have the username joecitizen@fastmail.fm, then email sent to
anything@joecitizen.fastmail.fm will be transformed to
joecitizen+anything@fastmail.fm and delivered to your account (it's
only available for Member level users and above).

This can be a very useful way of keeping track of email addresses that you hand out to different companies. For instance when signing up to a new web service, rather than giving out your regular email address, you give our companyname@joecitizen.fastmail.fm. If you start getting spam at companyname@joecitizen.fastmail.fm, you can just use the Define Rules screen to block that address.

One problem with this is that technically _'s (underscores) aren't valid in hostnames/domains (the part to the right of the @ symbol). So if your account was joe_citizen@fastmail.fm, then <anything@joe_citizen.fastmail.fm> is not technically a valid email
address. In many cases it will work, but for strict systems, they might reject the email.

There's now a work around to this. Simply replace the _ (underscore) with a - (hyphen). Eg use anything@joe-citizen.fastmail.fm. You should
only do this for sub-domain addresses where your username/alias has an _ in it. If you're using the regular joe_citizen@fastmail.fm address, do not replace the _ (underscore) with a - (hypthen).

2. Suppressing + address propagation on alias target addresses by adding +#noplus# on the target

If you have the account joecitizen@fastmail.fm and then create an alias such as joe@fastmail.fm that targets joecitizen@fastmail.fm, then if you send to joe+anything@fastmail.fm, we propagate the +anything part to the target of the alias, so the final destination address it joecitizen+anything@fastmail.fm.

This is useful when the target is a fastmail account, because the +anything is used to do fuzz folder matching to automatically file the message into a folder.

However if the target address is an external non-fastmail account, then this propagation may actually be annoying since it may result in an invalid email address that you didn't actually want to send to.

There's now a way to stop the propagation of the + component of an address to the target side of an alias, you need to pre-add a special +#noplus# component to the target of the alias. For instance taking the case above, if the target of the joe@fastmail.fm alias was joecitizen+#noplus#@fastmail.fm, then sending to joe+anything@fastmail.fm would send the email to joecitizen@fastmail.fm, rather than joecitizen+anything@fastmail.fm.