The example you quote showed an email originating in a Google (Gmail) data centre in sunny California (city of Mountain View to be precise) being subject to delay in delivery at a UK ISP data centre (ie Virgin Media), whereas there was no delay in delivery of that email from the MV Google (Gmail) server to whichever Gmail server you connected to in order to receive it (you don’t say which or where that is in the world – it might even have been in the same rack in the MV data centre).
1. The global email service is best described as collection of services which pass data to each other within a set of guidelines which are not legally enforceable because nobody is in total charge of the mechanics. There is no guarantee that email A can be delivered to recipient B within time C.
2. When it comes to self-protection against spam every ISP/email service either has a DIY in-house team or outsources it to an industry specialist. So, potentially – every ISP could be operating to differing sets of filter rules at any given point in time.
3. In this particular case, the sending Gmail server might be sending, say, a very large number of emails to A N Other Gmail server and both ends conforming to the same set of rules might be perfectly happy with that. However a non-Gmail server might regard the incoming email(s) as suspicious – for a variety of possible reasons – and reject wholly or defer delivery. The precise reasoning for that must obviously remain secret, hence descriptions such as “Policy Violation”.
5. Every ISP and email service provider strongly guards their anti-spam measures and will certainly not divulge them to any great extent in open forums such as this.
6. Note that California runs on PDT (Pacific Daylight Time) which is 8 hours behind UK time.