Use this method to create email messages in a mailbox such as Sent or
Draft.
$flags is an array of flags to be set to the $mail (if provided):
$flag can be one of:
- ANSWERED - message has been answered
- DELETED - message is marked to be deleted by later EXPUNGE
- DRAFT - message is marked as a draft
- FLAGGED - message is "flagged" for urgent/special attention
- SEEN - message has been read
This function automatically adds the '\' in front of each flag when
calling the server command.
Before calling this method, a connection to the IMAP server must be
established and a user must be authenticated successfully.