Today I didn't get email from my Debian server with cronjob results.
mail.err file showed the following:
Jul 12 00:23:16 server citadel: BDB: unable to allocate space from the buffer cache Jul 12 00:23:16 server citadel: cdb_fetch(2): Cannot allocate memory
Top showed that load is about 2 and citserv is using 102% of CPU, and a lot of processes - crontab+sendmail, crontab+sendmail... And the number of crontab and sendmail processes was groving. I tried to restart or stop sendmail. It didn't work - it was just like freezed. I tried to restart Apache. It took some 5 minutes.
I had to kill manually all sendmail and crontab processes. After that server load dropped to 0 and sendmail started to work.
The BDB error indicates an error in the Berkeley database subsystem which is used by Citadel for its storage. One option could be that you are running out of RAM, but it is also possible that the database structure itself corrupted.
One reported problem with Citadel is that the database system can get corrupted with an upgrade of the Citadel version without upgrading the database structure. Was there any recent update of Citadel installed on your server that you are aware of?