Dovecot isn't a mail server
You're looking at the wrong thing when you say "Dovecot to ignore....". Dovecot is a POP/IMAP client and doesn't really have anything to do with the attempt of Redmine to send emails to different users.
This functionality rests with Redmine itself and whatever mail server/frontend you're using (Sendmail, Postfix, etc.).
There's an issue that was opened 4 years ago on the Redmine website, titled: Better handling of unknown and aliased email addresses when submitting issue via email.
Your mail server
Lastly go through this guide on setting up email with Redmine and make sure you have it setup correctly (which you likely should). If'll notice towards the bottom of that page, there are directions for debugging email issues.
You'll likely need to configure what ever mail server you're using (Sendmail, Postfix, etc.) into sending these emails, but dropping the attempts to illegal addresses, or provide your mail server with the ability to ignore delivery attempts for a certain set of email addresses.
Blocking with Postfix
Here's an example that shows you how to go about blocking certain email addresses within Postfix. The article is titled: Postfix blacklist or reject an email address.
Open /etc/postfix/sender_access file
$ cd /etc/postfix
$ vi sender_access
Append sender email id as follows:
Save and close the file. Use postmap command to create a database:
$ postmap hash:sender_access
main.cf and add code as follows:
smtpd_recipient_restrictions = check_sender_access hash:/etc/postfix/sender_access
Save and close the file. Restart / reload postfix MTA:
$ sudo /etc/init.d/postfix restart