Marks messages listed in msg_number
for deletion.
Messages marked for deletion will stay in the mailbox until either
imap_expunge() is called or
imap_close() is called with the optional parameter
CL_EXPUNGE.
Note:
POP3 mailboxes do not have their message flags saved between
connections, so imap_expunge() must be called during
the same connection in order for messages marked for deletion to
actually be purged.