I am seeing errors in the Retain Worker log that read, "The server cannot service this request right now. Try again later." Why is that error occurring and what can be done about it?
The error indicates that either you have a throttling policy applied or the Exchange server is busy. Microsoft Exchange uses client throttling policies by default to track bandwidth for each Microsoft Exchange user and enforce bandwidth limits as necessary. Throttling policies should be turned off for the Retain Service Account, because they can affect the performance of Retain for Exchange when accessing mailboxes with a large number of folders and mail items.
See "Exchange Throttling Policy and Bandwidth / Performance (2010)"
See "Exchange Throttling Policy and Bandwidth / Performance (2013)"