[Mimedefang] Anybody any ideas what's causing these errors ?

[Mack]

I keep seeing these every now and then, 

Aug  4 11:27:58 xxxx sendmail[21826]: j74APxYT021826: from=<x at x.x.x>, size=625444, class=0, nrcpts=1, msgid=<6658066.1123149733542.JavaMail.x at x.x.x>, proto=ESMTP, daemon=MTA, relay=[x.x.x.x]
Aug  4 11:28:11 xxxx mimedefang.pl[13083]: TC: Logging : 1.881722 
Aug  4 11:32:58 xxxx sendmail[21826]: j74APxYT021826: Milter (mimedefang): timeout before data read
Aug  4 11:32:58 xxxx sendmail[21826]: j74APxYT021826: Milter (mimedefang): to error state
Aug  4 11:32:58 xxxx sendmail[21826]: j74APxYT021826: Milter: data, reject=451 4.3.2 Please try again later

Aug  4 11:33:30 xxxx mimedefang.pl[13083]: filter: j74APxYT021826:  append_html_boilerplate=1 
Aug  4 11:33:30 xxxx mimedefang[28593]: j74APxYT021826: smfi_chgheader returned MI_FAILURE
Aug  4 11:33:30 xxxx mimedefang[28593]: j74APxYT021826: smfi_replacebody returned MI_FAILURE
Aug  4 11:33:30 xxxx mimedefang[28593]: j74APxYT021826: smfi_addheader returned MI_FAILURE

The mail appears to be rejected at line 3 of the logs, but processing seems to continue for said mail

It seems to be on largish msgs, that it happens.
Any ideas? - should i increase the timeouts , they are at 600secs atm (ie MX_BUSY=600), however, it would appear the time is around 270secs when it times out.

TIA

Mack