Whilst investigating mail the queue I’ve put this quick guide together to help checking this again. My specific issue turned out to be not with the mail server being investigated, but with the mail server being sent to, these steps help quickly determine whats going on with your queue.
qshape deferred | head
This will show you the deferred mail – if this is stacking up you might be sending regularly to a mailbox that is having trouble.
This shows the current situation of your queue.
Both the above mirror the type of information you can get out of your control panel (e.g. plesk) quite easily.
To check the queue –
This is the one that should give the detail – look at the reasons for mail being queued as unlike the outline stats this command does show the reason. In my case this revealed that the receiving mail-server was reporting the mail-box as being full – despite the contrary being on that servers CP, a bit of tweaking on that server was required.
To reprocess mail in the queue –
Official documentation. very technical but gives deeper insight and worth a read.
5 Top tips for reviewing your postfix mail queue. has further detail on this including how to read / delete messages – we’re not interested in anything more than headers here.