message_size_limit question

I have a question about message size limits...We have a rather large size
limit on our mail server:

message_size_limit = 30000000

When users try to send messages that get up past 10 megs or so, sometimes
they generate an error in the logs like:

postfix/cleanup[7626]: warning: 62861490106: queue file size limit exceeded,
skipping futher client input.

Now I originally thought that such an error meant that someone tried to send
a message that exceeded the maximum allowable message size. However, the
behavior that I am seeing indicates otherwise. The users that are
experiencing this problem are trying to send to local users, not to external
mta's, so the message size should not be an issue. Also, they have plenty
of room left in their mailbox for an email of this size. Finally, we have
plenty of ram and disk space left for postfix to gobble up, should it deem
necessary to do so to receive the file.

I was wondering if due to the large attachment that the command_time_limit
setting could be causing cleanup to abort client input after the default
1000 seconds (command_time_limit=1000). This is just a guess though as I
have no idea why these messages are being rejected. :(

Any thoughts?

-j