I have many many years running Windows based servers but am trying my hand at linux.
I am running Debian 9 with:

Apache version 2.4.25
Bind DNS version 9.10
Dovecot version 2.2.27
Postfix version 3.1.8
SpamAssassin version 3.4.1

I am trying to configuring it through Webmin/Virtualmin

I can send mail out to a gmail account as well as other domains and it gets there. But when I send mail in from anywhere it is received by the system but cannot be retreived via a mail client either outlook or thunderbird.
I see the mail in the inbox via webmin but no joy retreiving it. What logs/conf files do i need to post to get some help.

Thank you in advance.

Since postfix works well allowing to send and receive emails, I guess you only need to setup dovecot as an imap server.
See this howto for more details (esp. the dovecot paragraph).

Regards