I’d split the whole process into the following steps and would suggest that you focus on getting each step working. This will also help you troubleshoot which step is actually the problem.
- mbsync setup for syncing email.
- postfix server for sending email
- mu setup for indexing email in synced folder. For this mbsync has to be setup first, and the email available to index.
- mu4e (comes with mu) for interface in emacs. If mu is setup, mu4e should work.