Is your email address correct? It should be email address of your pop3 email. In your case it is gmail.

Please see
https://www.shamimsplugins.com/docs/front-end-pm-pro/getting-started-2/pop3-feature/
http://support.google.com/mail/bin/answer.py?hl=en&answer=13273

Let me know.

No email client is needed.
I see in your screenshot that your email address is not in gmail domain. Please recheck if that email address is a gmail address.

If still not working let me know if you are comfortable editing php code. if i give you some debug php code, can you add that to further debug?

Can your website run cron?

Please follow step by step
1. Send a message from UserA to UserB.
2. Access UserB email and see if he received email notification. If yes, does that email subject contain [MESSAGE KEY-XXXXXX] ?
3. Reply to that email.
4. Access to your email address (which email you setup in POP3), Did you receive that email which is sent from UserB email ?
5. Wait at least 15 minutes then refresh your website couple of times.
6. Check if replied email from UserB imported in messagebox of UserA?

If any steps failed, let me know which steps it is.

If only step-6 failed, please go to class-fep-email-pop3.php and remove all // before error_log and again start from step-1 and after finishing all steps see your server error log and see any error there.

Let me know.

Please go to class-fep-email-pop3.php and add following code after error_log( ‘imap mail stream not available’ );

error_log( imap_last_error() );

Then try again and let me know error message.

It seems it is certificate issue. Please follow following steps
1. Setup your gmail for pop3 access. See https://support.google.com/mail/answer/7104828?hl=en
2. Allow less secure apps in https://myaccount.google.com/lesssecureapps for your same email address which you set as POP3 email.
3. Check Front End PM PRO > Settings > Emails > No validate certificates

Then try again and let me know.

Did you change error_log code what i suggested previously (remove // before error_log)? this code runs every after 15 minutes. So after 15 minutes refresh your website couple of times then see error log. When this code runs it at least show “pop3 event start” in error log if there is no error also.

Let me know.

Did you change any other settings in your website? (eg. debug code, cron settings)?

Please do following
1. add error_log( 'pop3 event init' ); just after function pop3_event() {
2. Go to Front End PM PRO > Settings > Emails > Email Piping/POP3 then set Enable as “None” then save changes and again set as POP3 and again save changes. Then wait at least 15 minutes and refresh your website couple of times then see error log.

3. If still no log shows please install any cron plugin (eg. https://wordpress.org/plugins/wp-crontrol/ ) then go to Tools > Cron Events and see if fep_pop3_event showing under hook name. If yes give me that row screenshot. Also click “Run Now” and see again error log if any log there.

Let me know.