[exim] why 550 does not result in removing mail from mailq?

Hello,

Beg your patience with me. I wonder why would the below rejection info
does not cause exim to give up trying to deliver to a non-existent
address?

2007-01-23 18:38:28 1H8EEP-00084l-5s SMTP error from remote mail server
after RCPT TO:<SOMEONE@???>: host naxpf012.abnamro.com
[155.181.177.27]: 450 <SOMEONE@???>: Recipient address rejected:
undeliverable address: host naipf012.sfb.na.abnamro.com[10.206.138.44]
said: 550 <SOMEONE@???>: Recipient address rejected: undeliverable
address: host aatryr001.sfb.na.abnamro.com[10.206.144.215] said: 550
SOMEONE@???... No such user (in reply to RCPT TO command) (in
reply to RCPT TO command)

Exim has at least 5 more emails in queue for this specific recipient and
it is trying to deliver them instead of giving up. Exim did not actually
bounce the particular message back to the sender. When I tried again with
this message id, I got to the point of waiting for data on socket. So I am
all lost. It seemed that 550 is no such user, but exim does not give up, I
try again and I am not greeted with 550 but the process of selection on
subprocess pipies takes a long time and eventually times out.

Other than adding today:

nolocaldelivery:
driver = dnslookup
condition = ${if match_ip{$sender_host_address}{192.168.16.2}}
transport = remote_smtp

just above the dns_lookup router, I did not make any other changes and
everything worked very well.

Can someone help? Thanks!

--
Zbigniew Szalbot