sendmail doesn't respond on heavy disk activity

[Marc Lucke <marc@xxxxxxxxxxxx>]

If I use dvdauthor and then telnet to port 25, the port opens but the 
Daemon doesn't answer until dvdauthor is finished.  Since it isn't a cpu 
related but a disk thing, renicing doesn't help.  I think it might be a 
libata thing.  How can I get my system to accept email when I'm using 
dvdauthor or otherwise heavily utilising my drive?