[exim] What is this in /var/spool/exim ?

I am finally curious enough to ask. I have a file that shows up in
/var/spool/exim that has the filename of the exim Banner string (ie.
smtp_banner = "${primary_hostname} ESMTP ${tod_full}" ).

# ls -l /var/spool/exim
total 204
drwxr-x---    2 exim     exim         4096 Oct 17  2004 db
-rw-r-----    1 exim     exim         1642 May 11 11:16 exim-process.info
drwxr-xr-x    2 exim     exim       118784 May 11 11:20 input
drwxr-x---    2 exim     exim        53248 May 11 11:20 msglog
-rw-r-----    1 exim     exim         1023 Apr 13 21:40 ${primary_hostname} ESMTP ${tod_full}

The actual contents of the file are log entries that would normally be
seen in exim_mainlog. Here are it's current contents (obviously I mangled
the email addresses and hosts for privacy).

2004-12-02 09:16:27 1CZsh0-0004a6-U4 => acou@??? <ACOU@???> R=internal_hosts T=remote_smtpH=host.doamin.tld [xxx.xxx.xxx.xxx]
2004-12-02 09:16:27 1CZsh0-0004a6-U4 Completed
2005-01-10 17:59:34 1Co9Rd-0005G2-G4 => ost1@??? R=internal_hosts T=remote_smtp H=host.doamin.tld[xxx.xxx.xxx.xxx]
2005-01-10 17:59:34 1Co9Rd-0005G2-G4 Completed
2005-01-13 20:34:49 1CpHIX-0000b3-98 => sha@??? R=internal_hosts T=remote_smtp H=host.doamin.tld[xxx.xxx.xxx.xxx]
2005-01-13 20:34:49 1CpHIX-0000b3-98 Completed
2005-03-22 05:11:22 1DDhI9-0003Jp-W8 => uval@??? <vice@???> R=internal_hosts T=remote_smtpH=host.doamin.tld [xxx.xxx.xxx.xxx]
2005-03-22 05:11:22 1DDhI9-0003Jp-W8 Completed
2005-04-13 09:37:35 1DLizn-0004Dn-NL => lric@??? <rich@???> R=internal_hosts T=remote_smtpH=host.doamin.tld [xxx.xxx.xxx.xxx]
2005-04-13 09:37:35 1DLizn-0004Dn-NL Completed

I can delete the file but it shows back up every so often. The file will
be written to every month or so and only 2 lines of log output when it
does occur.

I have had this occuring over every version of exim I have used over the
past 2 years and accross various linux distros (6 servers running exim). I
am currently on exim 4.44.

Just curious if anyone knows what causes this?

Kenneth