so squirrelmail is a webmail service.
There is a MTA (Mail Transfer Agent)/SMTP-Server is used for delivering mails from one system to another.
like: sendmail, postfix etc.
Then there is a MDA (Mail Delivery Agent) it delivers the recievied mails on the local system to the specified users.
like: procmail so that an email client like thunderbird mail or outlook or evolution can get the mails.
and there is an MRA (Mail Retrieval Agent) this is your email client see above.
and then you will have something like IMAP-/POP3-Server like: dovecot.
As I understand the sysad left the company w/o good documentation so we must figure out what you're using.
It should be reported in the log files. /var/log/messages and /var/log/maillog
It is not recommended to run kind of such service as root but who know what the sysad configured if it's not documented.
So login as root and do a
there you can see which jobs are running as root user.
maybe fetchmail is used to get the mails. So there could be somewhere a .fetchmailrc ... but he could have named it also "busfahrermails" this is for example the default fetchmail config. (.fetchmailrc)