0

No logging 

1

Major problems only; serious system failures and potential security problems 

2

Lost communications (network problems) and protocol failures 

3

Other serious failures 

4

Minor failures 

5

Message collection statistics 

6

Creating of error messages, VRFY and EXPN commands

7

Delivery failures (host or user unknown, and so forth) 

8

Successful deliveries 

9

(Default) Messages deferred (due to a host down, and so forth) 

10

Database expansion (alias, forward, and userdb lookups) 

15

Automatic alias database rebuilds 

20

Logs attempts to run locked queue files 

30

Lost locks