If you suspect that the email you have just received is spam, you can mark it as a junk email. In order to do this, click on Junk mail in the issue view.
Such e-mail will be then deleted, as well as automatically added as a blacklist filter on the mail server and mails from this e-mail will never be fetched again.
You can also move suspicious email addresses to blacklist, which is available in the Project Settings, Helpdesk tab. Emails coming from such emails will never be fetched.
Since version 3.0.9 of the Helpdesk plugin, the feature has been hidden from the Helpdesk settings interface. It was done because often customers click on this link by mistake. And there was no way to turn the changes back. So it was decided the link to be removed. It is recommended to use a blacklist in the project's settings and also mail server spam rules.
However, if you want to turn back the feature, please, open the file plugins/redmine_contacts_helpdesk/app/views/issues/_ticket_data.html.erb
There find the line:
%# link_to l(:label_helpdesk_spam)
And replace it with the following:
%= link_to l(:label_helpdesk_spam)
Please, restart your server and the junk link will be shown back.