Mail authentication fails with the following error in the
Jul 6 20:02:51 linux courier-pop3d: LOGIN FAILED, email@example.com, ip=[::ffff:192.168.133.219] Jul 6 20:02:51 linux courier-pop3d: authentication error: Input/output error
This is normal
courier-authdaemon behaviour when it not able to authenticate user due to invalid credentials.
The POP3/IMAP server says "Temporary problem, please try again later" when a bad password is entered.
authdaemond tries each of the configured authentication modules in turn, until either one accepts the login, or they have all rejected it (in which case the usual "Login failed" error is returned, and the user can try again).
However, if one of these modules is unable to run because some resource is not available, then it gives a
temporary failure response and no further modules are tried.
The error is more clear in Plesk 12.5.