Exim debug selector

Start an interactive debugging fake-session for lookups only (hence the -all):

exim -d-all+lookup  -bhc IP_ADDRESS

Selectors are as follows:

acl	ACL interpretation
auth	Authenication
deliver	General delivery logic
dns	DNS lookups
dnsbl	DNS black list code
exec	Arguments for exec()
expand	Details of string expansion
filter	Filter handling
hints_lookup	Hints data lookup
host_lookup	All types of name-to-IP address handling
ident	ident lookups
interface	Lists of local interfaces
lists	Matching things in lists
load	System load checks
local_scan	Debugging in the local_scan() function
lookup	General lookup code and all lookups
memory	Memory handling
pid	Add pid to debug lines
process_info	Setting info for the process log
queue_run	Queue runs
receive	General message reception logic
resolver	The DNS resolver’s debugging output
retry	Retry handling
rewrite	Address rewriting
route	Address routing
timestamp	Timestamp debug lines
tls	TLS logic
transport	Transports
uid	Change of uid/gid and looking up uid/gid
verify	Address verification
all	All of the above and also -v output

Taken from here