v6.20.02 fixes and new MDA_smtp destination

• merge #286 Add MDA_smtp destination for SMTP delivery
• fix #285, --idle exits 127 on untagged response before IDLE, as suggested, by looping untagged responses
• merge #284 from syahra712/fix/pop3-malformed-msglist to fix #277: make malformed POP3 UIDL/LIST abort only the affected account
• merge #279 fixing #278 crash when unsolicited IMAP FLAGS response precedes FETCH body (delete=true)