Using inbound SMTP with discussions requires that emails be
a) targeted at the forum using the email address and use a subject corresponding to a topic
b) targeted at the topic using the email address
c) targeted at a post using the email address
Any inbound email will result in a new post within a topic (in case 'a' potentially a new topic). It is not possible / supported (out-of-the-box) that messages are targeted directly at posts and result in direct responses.
Applying the aliasable aspect is not required technically - you can also address nodes using their node-dbid property value.
In any case you always have to make sure that the sender of the email is included in the allowed-senders configuration property (or you use a wildcard) and that their user has AddChildren permissions on the forum / topic.
Regards
Axel