Google does this in gmail. If I have an email address of user@domain.com, I want to be able to make lots of ad hoc email addresses that will arrive in the user@domain.com inbox just by adding a + after the user.

user+test1@domain.com
user+junkmail@domain.com
user+github@domain.com

...will all go to user@domain.com inbox.

https://gmail.googleblog.com/2008/03/2-hidden-ways-to-get-more-from-your.html

I had this functioning before my account was migrated to this platform and this feature was not available -- causing all of my + emails to stop working.

This allows for the on-the-fly creation of email addresses without needing to use the admin interface with aliases, etc.

This also allows for creating filtering rules on the TO address in my inbox, and other organizing tasks and automation.

several clients asked for such a feature which is available with "some" other service providers.

I'm interested to use subaddressing of emails in plesk soon! Thanks.

I have opened a support ticket regarding this. Plesk, your silence on this matter is unacceptable. This is now a STANDARD for most mail hosts. Please implement proper plus/sub addressing and expose settings for mail admins.

Please PROPERLY implement this in your Dovecot/Postfix configuration and expose options to the server admin:
allow plus addressing (turn on or off for server or domain - per domain is preferable)
allow auto-sort of plus addressed email (auto places into folders)
allow auto creation of new folders for plus addressed emails (creates non-existent folder)

This seems like it should be a simple feature to implement.
https://talk.plesk.com/threads/mail-alias-example-com-does-not-work-recipient_delimiter.353662/#post-867395

I can't have bounce messages silently discarded because the config file was overwritten. I have included a custom header in outgoing messages, but thousands of bounces don't include the original headers or message. So I have to process these manually. This is hurting our sender reputation.

Really important feature, any news on this?

Newsletter and CRM systems explicitly use subaddressing to monitor spam, for example https://www.mautic.org/docs/en/emails/bounce_management.html - so this is actually a very useful feature and should be implemented. Last comment seems to imply that a fix, found here https://talk.plesk.com/threads/postfix-not-delivering-messages-with-recipient-delimiter.293444/#post-720844 could be implemented easily.

Apparently in 17.0 main.cf already contains the line
recipient_delimiter = +
and it manages to rewrite an alias to the main address
but unfortunately postfix_local doesn't know about the delimiter:
postfix-local[pid]: cannot chdir to mailname dir username+extension: No such file or directory
postfix-local[pid]: Unknown user: username+extension
postfix/pipe[pid]: ID: to=<username+extension@domain.tld>, orig_to=<bounce+extension@domain.tld>, relay=plesk_virtual, delay=0.02, delays=0.01/0/0/0.01, dsn=2.0.0, status=sent (delivered via plesk_virtual service)
postfix/qmgr[pid]: ID: removed

The fix in https://talk.plesk.com/threads/postfix-not-delivering-messages-with-recipient-delimiter.293444/#post-720844 should be integrated in the default master.cf.
With the change mentioned there (${user}@${nexthop}), everything works, it does not break other behavior (putting user+ext into the folder ext in user's mailbox does not work anyway), whereas without the change, mails addressed to user+ext@domain.tld silently get dropped, which should NEVER happen.

The REAL needed thing is the ability to pass received messages to an executable, like cPanel allows to do. With this, you can even implement your own WeTransfer/Facebook/Asana in mail handler

In my opinion, catch all mail is enough. Spam never been an issue in mailboxes not published on webpages. There are spam filters at MX levels. Implementing VERP is not useful, catch all mail will suffice, implementing VERP it is not a solution to the spam

This is not only needed for VERP (for automatic bounce message processing), but also simply for address tags/plus addresses/subadressing (cf. RFC-5233 [1]) by real humans as a way of coping with the spam problem.
That's a kind of very simple to use "poor man's disposable email addressing".

When e.g. registering with some online service, provide an e-mail-address with a tag, which will be specific to this service. If that address leaks (or is sold by) that service, you have a track and then you just can reject/drop mails to that address. With tagged addresses, no need to set up a new address/alias/... - just invent whatever tag(s) you want and it's delivered to your base mailbox. (see for instance [2] or some better reference).

It's an easy way to cope with spam - more on a semantic level though...
You could even do some tag based filter rules (and even drop tag-less mails to rule out spammers that try stripping off the tags)...

Many hosters (MTA providers) support subaddressing since ages - they're obviously not running Plesk...

[1] http://tools.ietf.org/html/rfc5233
[2] http://www.kuro5hin.org/story/2003/3/24/22222/2368

Custom workarounds are good short term measures, but don't help long term when updates overwrite the config files. There is a feature request under review in regard to overwriting main.cf
https://plesk.uservoice.com/forums/184549-feature-suggestions/suggestions/4156106-allow-customization-of-etc-postfix-main-cf

http://stackoverflow.com/questions/17537661/how-do-i-get-a-plesk-11-server-to-accept-email-to-addresses-with-delimiter-verp