Redmine registration is currently not working due to mail configuration issue.
People can create accounts but the account activation mail is never sent.
If you have the mail in Maildir format:
$ cat ./the_mail_in_Maildir_format | dkimverify signature ok
With claws mail, open the mail, then click "File->Save Email as", then you can verify it with dkimverify manually:
$ cat saved_mail | dkimverify signature ok
Then in Redmine, go in Administration->Users and Select All in Status, and search for the mail address.
Make sure that there is only one email matching, and then, once the user is selected, you should see the following links on the top-right of the page:
Emails Activate Delete Users
You can then activate the account by clicking on "Activate".
Be aware that there is no confirmation and that once the button is clicked, the account is activated immediately.