To enable users to define their own external accounts to have Zimbra fetch mail from using Fetchmail, using the existing Zimbra LDAP would be brilliant.
The following article describes how LDAP can be used with Fetchmail:
Fetchmail - LinuxUserGroup Wernigerode
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Google Translate)
I hope the Zimbra developers will pick this up and add it, since it seems quite straight-forward and would be a very nice feature.