In addition to the authentication app option , Zimbra now has email as a TFA option. The unique code that was previously automatically generated by the authentication app will be sent to the email that was previously set up.

This email option as TFA is one of the new features in Zimbra 10.1. If currently your Zimbra 10 colleagues do not have this option, please upgrade to version 10.1 

To enable the feature via COS, go to the Zimbra Admin menu | Configure | Class of Service | Select the COS in question | Advanced. Enable email as a TFA option.

To activate on the user directly, go to the Zimbra Admin menu | Manage | Accounts | Select the account in question | Advanced. Enable email as a TFA option.

User login via webmail to activate TFA. Select Preferences | Accounts. Setup TFA by email. Enter email as the TFA option.

Then check the code sent to the email that has been entered as verification.

Enter the code in the box for verification.

Please logout and re-login to webmail. After entering standard authentication, you will be asked to enter the code sent to the email that has been setup as TFA.

Please try it and hopefully it will be useful