Re: Is it possible to run the inbound email when using HA?
Yes. You would only have to make sure that emails are routed to all servers, e.g. by setting multiple MX records in the domain, preferably with similar priorities to make it a fair / equal routing. When one server goes down, it should not accept any mail / SMTP connections and MTAs should automatically switch to one of the other servers. Also, if there is no response / a failure to transfer mails (e.g. because the server is in the process of failing), the MTA should try again and then also be able to pick one of the other servers.