My /var partition is full, what now?

In most cases this happened if the SMTP Proxy is active and configured as spam filter. In some circumstances, for example after certain types of mail bomb/floods the antispam daemon may fill up it's temporary directories with extracted mail contents or with quarantine files

In order to check if this happened, do the following in the a shell of your firewall:

du /var -hc --max-depth=4 | sort -rn | head -10

This shows you the directories with most content ordered by content size.

If you find amavis directories with a very high amount of content, you can remove old already processed files with the following commands:

rm -rf /var/amavis/virusmails/*
rm -rf /var/amavis/amavis-*
rm -rf /var/tmp/clamav-*

After this you can restart the smtp proxy by calling the following commands:

restartsmtpscan

and force the smtp proxy to process it's mail queue

postqueue -f

This happened to me also.
Tried the suggested solution but it only cleaned out about 100 MB.
I found out that it is ClamAV that fills the disk, with hundreds of /var/amavis/clamav-* files...
A lot of these files should be deleted, but I have no idea which one to delete.
The Endian logs also states that the virus engine is outdated, but I have no idea how to update it. Seems like the two issues might be connected somehow.
A userguide on how to remove old antivirus update files and also how to upgrade the antivirus engine might be a good candidate for a knowledge base article.