All users can use a great feature called "plus addressing", and paying users can use an extension of this called "subdomain addressing". They allow users to vary their email address. They also help users organize their mail into folders. These addressing features are great for managing mailing lists, registrations, and for avoiding spam by setting up reject filters for addresses that get on spammers' lists. You do not have to configure anything to create these addresses; they work right now. Note that subdomain as well as plus addresses work for usernames as well as aliases.
Plus addressing ('username+foldername@fastmail.fm') allows FastMail.FM users to create an unlimited variety of email addresses. By matching addresses to folders, these addresses can also help organize mail into folders.
Example: 'jcitizen@fastmail.fm' wishes to subscribe to a mailing list about sandwiches, but wants to use a different address. So, he subscribes to the list as 'jcitizen+sandwiches@fastmail.fm'. The mailing list fills his Inbox folder with mail. Eventually, jcitizen decides the mailing list is too much for his Inbox, so he creates a new Folder named 'sandwiches' (not 'inbox.sandwiches'). Thereafter, all mail he receives from the sandwiches mailing list gets organized into the folder 'sandwiches', not into the folder Inbox.
Caveat: The main downside of this approach is that addresses with '+' are considered (incorrectly) by many forms on the internet to be invalid. So entering an address in that form may not always work. Furthermore, people may find this kind of address hard to remember, because it is quite unusual. FastMail.FM subdomains, described below, overcome these problems.
Subdomain addressing ('foldername@username.fastmail.fm') offers all the advantages of plus addressing with fewer disadvantages. Paid accounts (Add free or better) have control of their very own subdomain.
Example: 'jcitizen@fastmail.fm' wishes to subscribe to a mailing list about sandwiches, but wants to use a different address. So, he subscribes to the list as 'sandwiches@jcitizen.fastmail.fm'. The mailing list fills his Inbox folder with mail. Eventually, jcitizen decides the mailing list is too much for his Inbox, so he creates a Folder in his account named 'sandwiches' (not 'inbox.sandwiches'). Thereafter, all mail he receives from the sandwiches mailing list gets organized into the folder 'sandwiches', not into the folder Inbox.
Caveat: your username cannot include an underscore ('_'), since hostnames on the internet cannot have underscores. Use the 'Rename' link on the 'Options' screen to change your account name if you want to remove the '_' to enable subdomains.
Note that only one level of subdomain addressing is supported. I. e., if your username is jcitizen@fastmail.fm, then anything@jcitizen.fastmail.fm will work, but anything@anythingelse.jcitizen.fastmail.fm will not work.
Note: Sub-domain and plus addressing also use fuzzy folder matching. When a regular folder match fails, folders are compared case-insensitively, with the characters '-'(hyphen), '_'(underscore), and ' '(space) treated the same. So if you have a folder "Mailing Lists.My List", then mails sent to username+mailing-lists.my_list@domain.tld will get filed to that folder.
Apart from the above, if after a match the delivery of the mail should have been to a subfolder, and if that subfolder is not present, the mail is delivered to the parent folder as you would expect, rather than to the Inbox folder. I. e., if you sent a mail to username+mailing-lists.yahoo@domain.tld, and if you don't have a subfolder "Yahoo" for the folder "Mailing Lists", the email would go to the "Mailing Lists" folder, rather than the "Inbox" folder.
This works with own domains as well - see Subdomains.
More sophisticated mail sorting can be done with sieve script. However, the power of sieve is proportional to the complexity of the sieve code you are willing to write.
Let's say your regular FastMail.FM address was user@fastmail.fm; if you have a folder in your online account named "friends", then sending an email to user+friends@fastmail.fm will cause that email to be filed in the user@fastmail.fm mailbox under the friends folder. For determining the proper folder, case is ignored and If the folder doesn't exist then the message is filed in the next higher level folder (in this case, Inbox). Sieve rules can overwrite this folder targeting, so plus addressing just gives the message a default folder to be filed into if nothing else intervenes when the rules start to run for that message. If you use a period in the address, then a subfolder is specified. This means that an email to user+friends.bob@fastmail.fm will be filed in the "bob" subfolder under the "friends" folder.
So if you use the Yes, swap: b+a.dom choice when setting up your Virtual Domain, then messages sent to friends@user.yourdomain.com will have their "X-Delivered-to:" address adjusted upon receipt to an "X-Resolved-to:" final address of user+friends@yourdomain.com, which will be filed in the "friends" folder of your user@yourdomain.com account. An email sent to friends.bob@user.yourdomain.com will be filed in your "friends/Bob" folder. If you have a space in the folder name, replace it with a hyphen ('-') when addressing. A message to sent-items@user.yourdomain.com will be filed in your Sent Items folder.