UIDs Cannot Start with a Dash

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In Messaging Server, UIDs may not begin with a dash character, "-". Why is that? It is because (1) Messaging Server's IMAP implementation uses a user's LDAP UID as their "identifier", and (2) Messaging Server supports the IMAP ACL extension, RFC 4314. The IMAP ACL extension expresses "negative" rights by prefixing an identifier with a dash character. As such, if Messaging Server allowed as distinct UIDs, the two UIDs "a" and "-a", then it would not be possible to tell if the ACL identifier "-a" meant to allow access to the UID "-a" or to disallow access to the UID "a". This ambiguity can only be removed by either not using UIDs as identifiers or by disallowing UIDs which begin with a dash. Messaging Server chooses to do the latter as the utility of having the identifier be the ACL is quite valuable. Not using the UID as the identifier is counter-intuitive and contrary to the industry norm.