On my RHEL5 systems, sendmail stopped working after I installed an update. The new version is sendmail-8.13.8-10.el5_11. I got errors like this in the logs:
NOQUEUE: SYSERR(nobody): can not write to queue directory /var/spool/clientmqueue/
and like this when I tried to mail from the command line:
WARNING: RunAsUser for MSP ignored, check group ids (egid=53, want=51)
can not write to queue directory /var/spool/clientmqueue/ (RunAsGid=0, required=53): Permission denied
It worked (for years) until the update.
The UID and GID for the smmsp user have to be 51/51. My system was set to use 53/53. Why, I don't know, silly.
To fix, I had to:
Redhat also says (paywall, sorry) that you have to have "mailnull" user set to uid/gid 47. That was already the case for me. It also warns about having duplicate uid/gid numbers, make sure that there's no other group or user 51.