Comment Re:So what exactly is it blocking? (Score 1) 145
Since SMTP allows forwarding by other servers this would require deep packet inspection.
If you mean the SMTP protocol supports chained delivery routes, then I do not think this is true (at least not used in practice). However, business customers of Gmail (at least) can request that a different SMTP server than Google's be used for outgoing mail, and (of course) anyone using an external mail client can send using any SMTP server they like.
I run a personal mail server and know I know just enough to know that I have vast chasms of ignorance about mail and network rules/firewalls.
Wouldn't it be easier to filter outbound packets destined for Gmail's SMTP servers and prevent Chinese email users from sending email to Gmail users? (This is an honest question.)