smtpd warn: not enough disk space

Hello,

just stumbled upon this. In /usr/src/usr.sbin/smtpd/queue_fs.c there is
a function fsqueue_check_space which checks for space based on
percentages. I am not using any partitioning on my laptop. Not a mail
server or anything special. One disk. / is that one disk. I know...

x500$ df -h
Filesystem Size Used Avail Capacity Mounted on
/dev/sd0a 105G 96.5G 3.2G 97% /

The check is working correctly. Capacity is at 97%. Still, there are
3.2G free disk space and smtpd is refusing to send out mail.

Jul 5 02:06:17 x500 smtpd[63712]: warn: not enough disk space: 4% left
Jul 5 02:06:17 x500 smtpd[63712]: warn: temporarily rejecting messages

What is the reasoning to check for disk space based on percentages? I
have never seen an application performing such kind of checks. If there
is not enough space, write would fail anyway. Checking for available
bytes before write would make sense. Checking a percentage value? I
don't get it. Not being able to send mail, although 3.2G space is
available makes no sense, IMHO.

Regards,
--
Christian