2.1.10 [RFC5545] Section 3.8.4.7, Implementations MUST Receive and Persist UID Values of at Least 255 Octets

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The specification states, "Implementations MUST be able to receive and persist values of at least 255 octets for this property."

On import, Outlook may not be able to distinguish UID values longer than 202 bytes when using message stores other than Personal Folders (PST) or Offline Folders (OST).