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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
firstname.lastname@example.org instead of his actual
email@example.com. In this case, he would
subaddress his email with
webshop (this part is also
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
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
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
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
will not end up in his
Archive folder. It will go
archive folder, if he has one, or
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
might specify his email to the hardware shop
firstname.lastname@example.org, 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
email@example.com set up in his
account, he could just as well hand out
firstname.lastname@example.org 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
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.