Where does SOGo prevent using those + extensions in your opinion?

It does not prevent me from creating aliases manually. But I think it would be great to have them without manual intervention. Did you have a look at the blog article about the Google Mail behavior?

Example: I have an address me@example.com. Someone can send mail to that address and it ends up in my inbox.
Problem: Mail sent to me+newsletter@example.com is not delivered, because no account exists for that address (nor did I create an alias).
Solution: Automatically deliver all mail sent to [account]+[anything]@server.tld to the inbox of [account].

Sorry, but that is not a function you have to activate in SOGo.
Check your SMTP and IMAP Server configuration.
You can activate there, that SMTP accecpts those emails, and IMAP puts those emails automatically in folder "newsletter" for address me+newsletter@example.com.

Example:
In Postfix set:
verp_delimiter_filter = +=

In dovecot set:
lmtp_save_to_detail_mailbox = yes
recipient_delimiter = +

Beware:

• The target folder has to exist before reception of such emails.
• This feature is case sensitive

this functiionnality is handled by the smtp server. Tipically postfix with the setting bellow:

recipient_delimiter = +

As Christian say, nothing in SOGo as imap client prevent using this address delimiter.

Alright, thank you very much for this clarification! It seems that I misunderstood what SOGo does and what it doesn't do...