SMTP protocol filter.
OT_SMTP_MAIL_OUTGOING - outgoing mail
Indicates object parts:
The objects contain a mail message in one stream.
When FF_SSL_TLS flag is specified, the library adds FT_SSL filter to a chain automatically after receiving STARTTLS command from mail client.
It makes sense to use FF_READ_ONLY_OUT flag, because the email clients usually have
small timeouts, and can break the transfer during filtering a large message in dataAvailable
event. If it is necessary to modify/block some outgoing messages, it is possible
to return "250 OK" or "554 Transaction failed" status to email client immediately
(using OT_RAW_INCOMING object), then retransmit allowed messages via different mail