This message essentially tells you that one of your users tried to send an email to an account that is handled by the mail1.mkuku.com server, but it failed. For each failed email, sendmail keeps it in its queue, and retries to send it later. The output of
mailq will display all of the emails currently in the queue and then you can delete them selectively from
/var/spool/mqueue (if you so desire).
There are two reasons as to why this failure occurs:
- The remote server's firewall is refusing connections from your server, or
- The SMTPd on the remote server has crushed.
You can test the connectivity with nmap to verify if the issue persists:
nmap -sS mail1.mkuku.com -p 25. If the port is not reported as open, you might want to contact the administrators of that domain to add an exception to their firewall.
As to if your server is sending spam, I'm afraid that I can't answer that for you. You should check your logs to see if all the mails are coming from the same sender, and analyze his/hers usage patterns. For example, you could see whether the emails are being sent in batches and whether the user logs in from a trusted network or for example, some "weird" ip in colombia.
PS. You might want to check projecthoneypot to see if your IP has been registered as a spammer.