Hello,
I want to create two separate projects "Service" and "Support" with mail address service@foo.com and support@foo.com.
Reading through your documentation and support groups, I stumbled on the following page:
https://community.atlassian.com/t5/Jira-questions/JIRA-Outgoing-email-address-based-on-project/qaq-p/228515
The last post explains, that this is possible by using aliases, so I could have one account, e.g. noreply@foo.com, retrieve all incoming mail and split by recipient, and when sending the "from" address will be accepted since it is an alias.
I have created such an email account with aliases, I can retrieve the mail correctly. I have configured the first project as follows (please disregard the authentication error):

I configured the email request for the second project as shown here:

After selecting the request type I cannot continue:

Btw: the "troubleshooting docs" link goes to a dead end.
Searching online for "Unable to create new mail channel due to connection duplication. Provided mail connection definition is currently used by another mail channel." I found this page: https://confluence.atlassian.com/jirakb/unable-to-create-new-email-channel-due-to-connection-duplication-error-827338480.html
However, when performing the queries as indicated, I do have entries in the table AO_54307E_EMAILCHANNELSETTING, so I'd guess it isn't the same issue.
I've backed up the system and the log files to be downloaded here:
https://www.dropbox.com/sh/gco11u1ersxtt94/AACiVoFGdLR_XS13cePywoFDa?dl=0