I’m not really sure I understand your questions correctly.
Helpmonks works as such that your shared mailboxes all have an internal unique email address (the one that you forward emails to) and the public email address, e.g. email@example.com.
When you reply to an email, the recipient sees that the email is coming from firstname.lastname@example.org and also replies to that email address.
To send emails over your SMTP-server, please go to the mailbox settings and change the SMTP-server to your own one.
In order to receive a copy of any reply from Helpmonks on your mail server, please set up a workflow and under actions set a CC or BCC email address.
I hope this helps.