I just realized I didn't reply to the list but to the OP only. Here is my reply:

CannotOverwriteExistingCassetteException: No match for the request (<Request (POST) http://localhost:9001/3.0/members/find?subscriber=%2Aexample.com%2A&list_id=foo.example.com&role=member>) was found. Can't overwrite existing cassette ('/home/hp/postorius_aditya/src/postorius/tests/fixtures/vcr_cassettes/ListMembersTest.test_search_members_1.yaml') in your current record mode ('once').

Is this error because the request is not present in test cases or is it some other error? Also, if it is a test case problem, can anyone suggest me how to change the test case (or where I could read about how to go about it) to make the build pass?

It's because the request is not recorded in the mentioned yaml file. In order to record tapes, you will have to have a running mailman instance. The config file for mailman can be found in the testing directory of postorius.

1. Make sure you have an empty mailman database (just remove everything that mailman creates ^^).
2. You will want to delete the yaml file you want to overwrite.
3. run tox -e record in postorius.

Repeat steps 1-3 as long as you get errors (you will have to fix the errors of course)

4. confirm that everything works by running tox

commit the yaml file you wanted to overwrite.

Sidenote: You might not have to rerecord the yaml file over and over if you are only fixing things that don't result in new/changed requests.