Beginning with V8.12 sendmail, vacation logs all its error and warning messages via the syslog(3) facility (Section 14.3). Warnings are logged at LOG_NOTICE, and errors are logged at LOG_ERR.

The syslog facility reports them like this:

Mar  1 13:30:05 lady vacation[26884]: vacation: can't open mailbox database: Service 
unavailable.

If you prefer to have these errors and warnings printed to your screen, you can use this -d command-line switch. It is better used outside your ~/.forward file because otherwise, printed errors will be sent to sendmail where they might be lost. You use the -d command-line switch like this:

% vacation -d bob < /dev/null
vacation: no such user bob.

Here, the administrator is about to set up a ~/.forward file for a user, and tests the vacation command with this -d switch. Because the user is bbob, and not bob, the error is immediately evident. Without the -d, the error would have been logged, and that log message might have been sent to another dedicated logging host.