From the version 9 SRND, you will find:
Per RFC 3261 (section 19.1.4, URI Comparison) comparison of the userinfo of SIP URIs has to be case-sensitive. According to this standardized behaviors, Alice@cisco.com and email@example.com are not to be considered equivalent. When routing directory URIs, Unified CM respects this standard and looks for a case-sensitive full match of the user portion and a case-insensitive match of the host portion. To avoid confusion, Cisco highly recommends provisioning only directory URIs with all lowercase userinfo so that all directory URIs can reliably be dialed by entering all lowercase information. Unified CM 9.1 and later releases can be configured to always use case-insensitive comparison of the user info portion of directory URIs. This can be achieved by configuring the enterprise parameter URI Lookup Policy accordingly. This setting applies to matching locally configured directory URIs and also to matching directory URIs for which an ILS lookup is done. The default setting of this enterprise parameter defines standard compliant case-sensitive matching of the user info portion of directory URIs.
Cisco is apparently doing the correct thing by considering Me@abc.com and firstname.lastname@example.org two different things. Unfortunately I can't imagine anyone in the wild assuming the same thing. Regardless of whether an enterprise was lucky enough to develop and follow a procedure where all mail IDs or SIP URIs were entered with a consistent case, lower, upper or camel, they can not assume their users or clients will do the same.
With that, starting in version 9 there is an enterprise parameter that will toggle how lookups are done.
Navigate to System | Enterprise Parameters and in the Enterprise Parameters Configuration section, find URI Lookup Policy.
|URI Lookup Policy|
From the help file:
This parameter specifies the way the match is done for the configured user part of the URIs in Cisco Unified Call Manager. If the parameter is set to Case Sensitive, we will match the exact case sensitive URI configured in Cisco Unified CM. If the parameter is set to Case Insensitive, then case insensitive lookup will be done.