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Prefetch information about POP3 messages.
If the FetchProfile contains UIDFolder.FetchProfileItem.UID
,
POP3 UIDs for all messages in the folder are fetched using the POP3
UIDL command.
If the FetchProfile contains FetchProfile.Item.ENVELOPE
,
the headers and size of all messages are fetched using the POP3 TOP
and LIST commands.