This method supports unique IDs instead of message numbers. See
{@link ezcMailImapTransportOptions} for how to enable unique IDs
referencing.
The message number $msgNum must be a valid identifier fetched with e.g.
{@link listMessages()}.
The message is not physically deleted, but has its DELETED flag set,
and can be later undeleted by clearing its DELETED flag with
{@link clearFlag()}.
Before calling this method, a connection to the IMAP server must be
established and a user must be authenticated successfully, and a mailbox
must be selected.