Why can't I POP from one Gmail account to another? [closed]

I guess Google considers their own client 'insecure'. Try allowing access for 'less secure apps': https://www.google.com/settings/security/lesssecureapps

edit (2019-12-17):

As this is a popular answer, I want to use this to warn people running into this problem that the solution presented here will likely soon stop working.

Yesterday Google communicated the following to G Suite administrators:

We’re constantly working to improve the security of your organization’s Google accounts. As part of this effort, and in consideration of the current threat landscape, we’ll be turning off access to less secure apps (LSA) — non-Google apps that can access your Google account with only a username and password, without requiring any additional verification steps. Access through only a username and password makes your account more vulnerable to hijacking attempts. Moving forward, only apps that support a more modern and secure access method called OAuth will be able to access your G Suite account.

Access to LSAs will be turned off in two stages:

1. June 15, 2020 - Users who try to connect to an LSA for the first time will no longer be able to do so. This includes third-party apps that allow password-only access to Google calendars, contacts, and email via protocols such as CalDAV, CardDAV and IMAP. Users who have connected to LSAs prior to this date will be able to continue using them until usage of all LSAs is turned off.
2. February 15, 2021 - Access to LSAs will be turned off for all G Suite accounts.

I got this fixed. The answer for me was in that link that we all received. I thought I followed it and probably did but not with the secondary account that I wanted to add.

So this is what I did:

1. Logout of your primary gmail account.
2. Login to the secondary gmail account (the one you wanted to add).
3. Go to https://accounts.google.com/b/0/DisplayUnlockCaptcha and follow the instructions on the page to allow access.
4. Logout of this account.
5. Log back into your primary gmail account and now try adding that second account. It should work now.
6. You should get an email to confirm that you own that account. Make sure you login to that second account again to confirm (by clicking on the link or entering the confirmation code).

It worked perfectly for me. I have two gmail accounts synced into one now :)

^{Source: Can't add pop3 account in gmail account at gmail forum}