Discovered that our customers with Ampersand (&) characters in their email address (i.e. a&b@example.com) can login to webmail just fine but an Administrator cannot impersonate these users. Attempting to Impersonate User for one of these addresses will result in "User Not Found" errors.

 

A temporary workaround to this issue is to Percent-Encode the address in the Impersonate User field (i.e. a%26b@example.com).

 

SmarterMail Enterprise version 12.3.5318.