Plus addressing and subdomain addressing

Plus addressing and subdomain addressing are features that allow you to give out variations of your normal email address that still arrive at your account. It's easy to create a rule to automatically filter messages sent to a particular variant, which can be helpful for managing mailing lists and site registrations.

You do not have to configure anything to create these addresses. They work right now, for both your username@domain.tld address and any aliases you have configured, both on Fastmail domains and any personal domains.

Plus addressing

Plus addressing means any email sent to username+whateveryoulike@domain.tld is still delivered to your account. The part of the email address between the "+" and the "@" is ignored for determining which account it belongs to. This allows you to create limitless variations on your email address to give out to different people, sites or mailing lists.

If the part after the "+" matches the name of one of your folders (see below for how the matching works), the message will automatically be delivered there instead of your Inbox. You don't even need to create an explicit rule!

For example, suppose you gave someone the address username+hiking@domain.tld. Any messages sent to this address will be delivered to username@domain.tld, and if you have a folder called Hiking, will be delivered straight to that folder rather than your Inbox.

The main downside to plus addressing is that addresses with a "+" are considered (incorrectly) to be invalid by some websites, and disallowed on some registration forms. Also, people may find this kind of address hard to remember, because it is quite unusual. Subdomain addressing, described below, overcomes these problems.

Subdomain addressing

Subdomain addressing offers all the advantages of plus addressing with fewer disadvantages. It works the same as plus addressing, but instead of username+whateveryoulike@domain.tld, you can use an address of the form whateveryoulike@username.domain.tld. Just like with plus addressing, messages will be automatically filed into folders with a matching name.

Please note, your username cannot include an underscore ("_") or a dot (".") if you wish to use subdomain addressing. Hostnames on the internet cannot have underscores and could be or if both domains exist. You can change your username on the Advanced → Rename screen.

Only one level of subdomain addressing is supported: whateveryoulike@username.domain.tld is fine, but somethingelse@whateveryoulike.username.domain.tld will not work.

How a folder is matched

Sub-domain and plus addressing use fuzzy folder matching. When a regular folder match fails, folders are compared case-insensitively, with the hyphen ("-"), underscore ("_") and space (" ") characters treated the same. A full stop (".") is used to separate a parent from a subfolder. So if you have a folder "Mailing Lists.My List", then messages sent to username+mailing-lists.my_list@domain.tld will get filed into that folder.

If after a match the delivery of the mail should have been to a subfolder but the subfolder does not exist, the mail is delivered to the parent folder, rather than to the Inbox. For example, if you sent a message to username+mailing-lists.hiking@domain.tld and you don't have a subfolder "Hiking" under the "Mailing Lists" folder, then the email will go to the "Mailing Lists" folder rather than the Inbox.

If you create a rule that files the message into a folder, this will override the folder matching described above.

Sending email from a plus or subdomain address

To send email using whateveryoulike@username.domain.tld, create an account identity with that email address. Alternatively, create an account identity with the email address *@username.domain.tld which will let you send email with any text you like replacing the *.