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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 email@example.com, then email sent to firstname.lastname@example.org will be transformed to email@example.com 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 firstname.lastname@example.org. If you start getting spam at email@example.com, 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 firstname.lastname@example.org, 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 email@example.com. You should only do this for sub-domain addresses where your username/alias has an _ in it. If you're using the regular firstname.lastname@example.org 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 email@example.com and then create an alias such as firstname.lastname@example.org that targets email@example.com, then if you send to firstname.lastname@example.org, we propagate the +anything part to the target of the alias, so the final destination address it email@example.com.
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 firstname.lastname@example.org alias was email@example.com, then sending to firstname.lastname@example.org would send the email to email@example.com, rather than firstname.lastname@example.org.