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'Plus' (+) subaddressing

AltMail.se implements so-called '+' (plus) subaddressing. This allows you to seamlessly sort incoming email from various sources into different folders. Subaddressing your email address means appending a label to your account name, separated with a plus sign. For example, when prompted to enter his email on a webshop's checkout page, John Doe might enter john.doe+webshop@altmail.se instead of his actual email john.doe@altmail.se. In this case, he would subaddress his email with webshop (this part is also called the detail of the address).

Here's how the sorting works. If there exists a folder exactly matching the lower-case "detail" of the recipient address, the message will be delivered to that folder. Otherwise, the message will be delivered as normal (placed into the INBOX unless classified as spam, etc).

Simply put, your account should have existing folders for those subaddresses for which you wish to auto-sort incoming email. Creating this folder is the only piece of setup required for the filtering to work, and it is not even necessary to do this in advance. In our example, John might wait for the first email to arrive from the webshop. He might move this email from his INBOX to a new folder called webshop, creating the folder with his mail client in the process, thereby enabling subsequent emails from the webshop to land there. But given how pedantic he is, he probably created the folder right away when he gave out his subaddressed email.

One caveat is that the subaddress part (or "detail") is converted to lowercase before attempting to resolve it to an existing folder. For this reason, email sent to john.doe+Archive@altmail.se will not end up in his Archive folder. It will go to his archive folder, if he has one, or his INBOX.

Due to the way IMAP folder structures work, it is even possible to direct incoming email to a subfolder in a nested structure by encoding the folder path separators as dots. For example, if John has a subfolder called hardware under webshop, he might specify his email to the hardware shop as john.doe+webshop.hardware@altmail.se, and emails sent to that address will be delivered to the correct subfolder.

Subaddress or alias? You can have both!

You can freely mix subaddressing (designed for easy auto-sorting) with the usage of aliases (aimed to preserve privacy). That is, if John had an alias nowhereman@altmail.se set up in his account, he could just as well hand out nowhereman+webshop@altmail.se as his address towards the webshop. This would provide him with an easy way to sort incoming messages into the appropriate folder, while also hiding his real email address from the counterparty.

The main advantage of subaddresses is the simplicity of their setup. No separate trip to the account portal is required, nor is there any manual fiddling involved to set up filter rules. Also, there is practically no limit on the number of subaddresses one might be using at any moment. All this makes subaddressed variants extremely cheap to invent and use.

The downside is that subaddresses do not come with any privacy protection: the "bare" address is trivially recovered, automatically even. So bad actors can easily target your "real" address. Also, removing the corresponding recipient folder will not stop senders from reaching you – in fact, their email will now end up right in your INBOX. For this reason, we recommend that you always use aliases (subaddressed or not) when corresponding with all but the most reputable parties.

Unfortunately, there are places on the web where one simply cannot enter a subaddressed email, as the '+' symbol is (wrongly) considered invalid. In such cases, it is best to stick to a burner alias.