The server denied POP3 access for the specified username and password.

Hello
For Gmail, you should:
1) Enable POP/IMAP in your account settings
mail.google.com -> Settings -> See all settings -> Forwarding & POP/IMAP tab, Enable POP/IMAP.

2) Manage account access security (Google in general)
myaccount.google.com -> Security -> Your account's security activity

Cheers
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