kmail - sendmail problem

[plutonium239]

hi
i have a problem with sending mails. i use kmail and send mails via sendmail.
everything worked fine till yesterday, and i cannot remeber that i have changed any configuration

when i try to send email following error message is returned by kmail:
Sending failed:
Sendmail exited abnormally.
The message will stay in the 'outbox' folder until you either fix the problem (e.g. a broken address) or remove the message from the 'outbox' folder.
The following transport protocol was used:
Sendmail

in var/log/maillog there is following error message
Dec 16 11:16:30 localhost sendmail[4278]: jBGAGTc2004278: SYSERR(root): putbody: write error: No space left on device
Dec 16 11:16:30 localhost sendmail[4278]: jBGAGTc2004278: SYSERR(root): Error writing control file qfjBGAGTc2004278: No space left on device
Dec 16 11:30:47 localhost sendmail[4422]: jBGAUl0B004422: low on space (SMTP-DAEMON needs 340 bytes + 100 blocks in /var/spool/clientmqueue), max avail: 0

when i switch the MTA to postfix it looks like mails would be sent normally but they just get lost and i get this message in var/log/maillog
Dec 16 11:10:53 localhost postfix/cleanup[3517]: warning: 41CCEBA32D: write queue file: No space left on device
Dec 16 11:10:53 localhost postfix/pickup[3480]: warning: maildrop/71134C06F1: Error writing message file

receiving mails works fine.
does anyone know what is meant by this error msg?

thx