Sometimes you want to give the same pool of IPs to different apps/users of your smtp relay service. You may want to give one set of credentials to marketing department, another set of credentials to billing department. Both department will share the same IP pool aka virtualMTA pool. Here is an example configuration:
# start userlist (authenticated with SMTP AUTH protocol)
<smtp-user username1>
password changemetosomethingreallysecure1
source {authorized_users}
</smtp-user>
<smtp-user username2>
password changemetosomethingreallysecure2
source {authorized_users}
</smtp-user>
#end userlist
<source {authorized_users}>
always-allow-relaying yes
process-x-virtual-mta yes # allow selection of a VirtualMTA based on 'x-virtual-mta' header
max-message-size 0 # no size (in byte) restriction on submitted messages
smtp-service yes # present SMTP service to these users
require-auth true
default-virtual-mta mta-pool1
</source>