POP Peeper FAQ
Does POP Peeper work with two-step verification (Hotmail, GMail, Yahoo)?
If you enable two-step verification process on your account, POP Peeper will only be able to access your account if you use IMAP or POP3. When using IMAP, it is recommended that you set POP Peeper to use OAuth2. Otherwise, when using IMAP or POP3 with two-step verification, you will need to create a unique app password (see links below for more information). If POP Peeper is accessing your account using Webmail (ie. the Server Type is "Hotmail/Outlook," "GMail," or "Yahoo"), POP Peeper will not be able to access your account so it is recommended that you disable two-step verification or use POP3/IMAP.
As of Webmail v188.8.131.52, GMail accounts that are detected using 2-step verification will be suspended in order to prevent excess verification checks. Depending on the version of POP Peeper that you're using, you may get a specific error or POP Peeper may report a -9000 or -9001 error code. You should correct the problem (disable two-step verification or switch to POP3/IMAP), then you may need to reset the error (click on the yellow '!' button in the status bar and press the "reset" button) to remove the suspension; the account will be suspended again if 2-step verification is still enabled. Note that exiting and restarting POP Peeper will also lift the suspension, so it's important that you fix the problem as soon as possible in order to avoid verification checks from the webmail service.
For more information about setting up a unique app password: