Today I noticed that no xtra 'Sent' emails were showing on the Apple Mail app on my iPhone. Checked the server settings as well as I could given my limited knowledge and confirmed that they are the same as on my iPad, which is operating just fine.

I called Spark who got me to re-do all the IMAP server settings etc without success and then they gave up and put me through to Apple (I think in Oz). 

Apple did much the same thing and then escalated it to a more techy person (I think in the US).

After 90 minutes total on the phone, Apple came back after a long on-hold and told me that the behaviour was 'as expected'. I.E. If you send an email via xtra  from a Macbook or an iPad using Apple Mail, it will show up in other devices' Sent folders - but not on the iPhone Mail app 'sent' box. Only emails sent from the iPhone itself will show up in the iPhone Mail 'sent' box.

Similarly, emails sent from an iPhone show in the phone's Sent box but not other Apple devices' Sent boxes.

This may well totally correct behaviour but it strikes me as odd (because the other devices do sync) and I don't know enough to tell whether I'm being fobbed off. 

The only way to see all sent emails on an iPhone, regardless of sending device, is via Spark xtra webmail.

Would welcome comments and advice.

TLDR: iPhone Mail app does not show Sent messages sent on other Apple devices' Mail apps. T/F?