One interesting thing I learnt this week from a mailing list that I knew how it worked but didn't know the detail of was the OOF history feature. This feature has been around for ages and its what Exchange uses to ensure you don't receive more then one copy of an OOF message when you send to a mailbox where the OOF status is enabled. According to this KB https://support.microsoft.com/en-us/help/3106609/out-of-office-oof-messages-are-sent-multiple-times-to-recipients this list has a limit of 10000 entries and can cause problems at times like any feature so it give some details on how to manually clear it. The more interesting part for a developer is the property they mention PR_DELEGATED_BY_ RULE (or PidTagDelegatedByRule https://msdn.microsoft.com/en-us/library/ee218716%28v=exchg.80%29.aspx ). This property contains a list of all the Email Addresses that this Mailbox has sent an OOF message to while the OOF feature was enabled which is som...
Pushing the Envelope in Messaging and Office 365 Development