Lookup domain for incoming email
By default, Virtualmin's lookup-domain-daemon process will launch a
separate subprocess to find the domain and spam settings for each incoming
message on your system. However, this can consume large amounts of CPU time
and memory if multiple messages arrive at the same time.
To have email processed one by one instead, select the One at a time
option for this field. Mail delivery will be slightly slower, but CPU and
memory use will be limited.