Sendmail will use by default a single mail queue. This is what most users will need, and if you don’t have any special requirement you will not care about this. Still for high traffic mail servers it might be useful to split the queue over several directories, as thousands of files in a single directory will become a performance penalty at some point and also processing the queue sequentially will become very slow. This post will show how we can implement multiple mail queues with modern sendmail versions.

Let’s start by assuming we want to use 8 mail queues. First thing is to create the actual directories as sendmail will not do this by default:
mkdir /var/spool/mqueue/q{1,2,3,4,5,6,7,8}

And fix the permissions to the ones of the original folder /var/spool/mqueue. For ex. this might look like:
chown -R smmta:smmsp /var/spool/mqueue/q*

using a default sendmail install running on debian. Fix the users to the specific ones found on your system (ls -al /var/spool/mqueue if you are uncertain of this).
Next, we need to enable the multiple queues in the sendmail configuration. For this we will edit sendmail.mc (normally found under /etc/mail) and append one line:

define(`QUEUE_DIR’, `/var/spool/mqueue/q*’)dnl

and now regenerate sendmail.cf; this is done normally running:
m4 sendmail.mc > /etc/mail/sendmail.cf

(fix your paths appropriately), or if you are using debian sendmail you can just run make all in /etc/mail.
After restarting sendmail, it will start using the multiple queues we defined. Running mailq will output each of the queues:
mailq
/var/spool/mqueue/q6 is empty
/var/spool/mqueue/q4 is empty
/var/spool/mqueue/q3 is empty
/var/spool/mqueue/q2 is empty
/var/spool/mqueue/q5 is empty
/var/spool/mqueue/q1 is empty
/var/spool/mqueue/q7 is empty
/var/spool/mqueue/q8 is empty
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Thanks
Manoj Chauhan