Redmine

Could you check again: I'm using redmine r1350, and checking with gmail I have correctly mymail+redmine@gmail.com, but gmail show it as mymail@gmail.com (see the "display original" in gmail, line starting with Delivered To:)

I can't reproduce. The email address is not broken when used as the From email address.

Here is the raw email that is generated. You can see the From address is example+somthing@gmail.com:

From: example+somthing@gmail.com
Bcc: jsmith@foo.bar
Subject: Redmine test
Mime-Version: 1.0
Content-Type: multipart/alternative; boundary=mimepart_48185f4af03c0_cac24164581
10
X-Mailer: Redmine
X-Redmine-Site: Redmine
X-Redmine-Host: www.redmine.org

--mimepart_48185f4af03c0_cac2416458110
Content-Type: text/plain; charset=utf-8
Content-Transfer-Encoding: Quoted-printable

[...]

In your case, how do you see that somthing@gmail.com is used rather than example+somthing@gmail.com ?
You can run redmine with development environment and check the raw email dump that you will find in the log.