MailEnable Standard Guide
IMAP - Settings
Services and Connectors > IMAP Service > IMAP - Settings

Setting

Description

Enable SSL/TLS support Enables SSL and TLS support for the IMAP service.
Allow clients to login using PLAIN authentication Enables PLAIN authentication for the IMAP service.
Force clients to login securely (over SSL) Users are required to use SSL or TLS to authenticate.
Enforce mailbox quotas When users copy messages up to the server via IMAP this will make sure that they do not exceed their quota and return an error message.

Enable NTLMv1 authentication

If enabled then secure authentication between the server and the supported client is enabled.  This will allow the server to accept requests from the client to use secure transmissions for the authentication method.  The client also has to be enabled use this secure authentication. For example, in Outlook the feature is called SPA – Secure Password Authentication.  More information on NTLM can be found in the Overview of NTLM authentication. NTLMv2 is not supported.

Enable CRAM-MD5 authentication

CRAM-MD5 Challenge-Response Authentication Mechanism is intended to provide an authentication extension to IMAP4 that neither transfers passwords in clear text nor requires significant security infrastructure in order to function. Only a hash value of the shared password is ever sent over the network, thus precluding plaintext transmission.  

Enable XLIST support This option allows the IMAP service to tell the email client what folders are the Junk, Deleted Items, etc. Since clients vary the name of these, this helps the client match it what the server has configured.
Advertise mailbox quotas to IMAP clients This will inform IMAP clients what the quota is for the mailbox. Some clients, such as Thunderbird, will show this to the user. This does not enforce the quota though, so a client may ignore this or not support it. The Enforce mailbox quotas option is required still in order to stop them going over quota.