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.
chown -R smmta:smmsp /var/spool/mqueue/q*
m4 sendmail.mc > /etc/mail/sendmail.cf
Note: if you want to add more folders to the configuration all you have to do is to create the respective folders, set the appropriate permissions and restart sendmail.
If you had existing mails in the queue (most likely if you were looking for this solution), if you want them still processed, move them from /var/spool/mqueue in one of the newly created queues (q1 for ex).
Individual queue directories can be symbolic links to other partitions to spreads load among multiple disks. Queue IDs are unique across queues so you can move the items among queues if you have to.