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.