Alessandro Raffa

In some cases you want your wordpress users to login with their email address instead of the username. To do this, you can use a good plugin like ‘WP Email Login’ or you can activate a ‘Register Plus Redux’ plugin’s option, but when the users will ask for lost password, the old / original username will be emailed to them, and this can cause confusion. To avoid this situation, just install and activate Login Email Sync, and it will sync the username ( user_login database field ) with the email ( user_email database field ) every time a user updates her/his profile.
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Login Email Sync

In some cases you want your wordpress users to login with their email address instead of the username. To do this, you can use a good plugin like &#03 …

imsoftware

This plugins replaces the regular authentication of WordPress and offers instead the authentication of users via IMAP accounts. Password reset and retrieving are suppressed in order to avoid confussion.
The IMAP authentication server as well as an automatic padding of the domain to a user name can be set on the plugins setting page under Settings > IMAP Authentication. Furthermore, users having a IMAP account on the defined server can automatically be register if set so.
Contributors: Aaron Parecki, Riccardo Magliocchetti, Lorenz Schmid, Marius Müller

WP IMAP Auth

Use IMAP accounts to login in to WordPress. Replaces the regular authentication.

Login Email Sync vs WP IMAP Auth