[Exim] Mails stay in queue longer than retry allows?

Hello,

this is the last retry rule, and it does match some mails I see in
my queue:

*               *           F,6h,15m; G,24h,1h,1.5; F,7d,6h

Exim appears to keep some mails longer, though, if delivery fails in
odd ways:

== sales@??? R=outgoing T=t_outgoing defer (0): SMTP error from remote mailer after end of data: host mail.baoshchina.com [221.136.64.179]: 850 System is busy. jDQnABBPUEDSHTKE.1(26)

I see all kinds of odd codes, 850 being just one example, and it's always
those mails which I end up removing manually. Do I need to change my
retry rule or is this a bug?

Michael