[exim] Exim + Dovecot deliver overquota problems

Good morning,

We are having an issue with a dovecot + exim setup.
We are using dovecot's deliver for the mail delivery with the pipe driver:

dovecot_transport:
driver = pipe
command = /usr/lib/dovecot/deliver -d $local_part@$domain -f $sender_address
user = vmail
group = vmail
envelope_to_add = true
return_path_add = true
delivery_date_add
log_output
temp_errors = 64 : 69 : 70: 71 : 72 : 73 : 74 : 75 : 78

In dovecot.conf I have enabled the quota_full_tempfail, so the deliver
script fails when the final recipient is overquota.
quota_full_tempfail = yes

This is working without problems with all the mail except for the
overquota recipients. On those recipients I get:
Child process of dovecot_transport transport
returned 75 (could mean temporary error) from command: /usr/lib/dovecot/deliver

I would like this to be returned as a 550 error (or at least a 450
temp error), but I get the standard success message:

helo example.com
...
250 Accepted
data
354 Enter message, ending with "." on a line by itself
jlk
.
250 OK id=1NMgHz-0004VH-Jw


Thanks for your help