(1) Switch sendmail to postfix.
sendmail config is for local delivery only. That means
mail local-username will work. Sending anything with a domain name will not work. It will NOT send to external email address. It will NOT receive any email from external.
sendmail is a solid email server package. However, due to its cryptic configuration format, less people are using/learning it.
exim are the norm now.
(2) Install Dovecot
postfix is for sending and receiving email. To pick up email remotely with pop3/imap4, you need
(3) Advice for Webmin
DO NOT MIX WEBMIN WITH MANUAL CONFIGURATION. <-- Repeat that 3 times!!
Webmin save all your settings in its own database, then write them to configuration files of different packages(eg. postfix, dovecot, etc). Every time you make a change with
webmain and hit save/apply, it rewrite the configuration file.
For example, look at the following sequence
webmin to config
postfix configuration file by hand
webmin to change
All changes in step 2 are lost.
If you are going to use webmin, configure everything with webmin, do the webmin way. No mixing!