It is always best practice to build .cf file from .mc

m4 sendmail.mc > /etc/sendmail.cf

when you modify /etc/mail/aliases.
Possibly you see following message.
alias database /etc/mail/aliases.db out of date

Simply run following and will fix it.

newaliases

OR
run 'make' in /etc/mail to update all databases
OR

sendmail -bi

Following messages you see in log and mail.

• DSN: unknown mailer error 126
• stat=unknown mailer error 126
• runscript.sh: Permission denied
• 554 5.3.0 unknown mailer error 126

It is because script you call to catch email OR execute when received has permission issue.
Simply set permission on that script to “Sendmail Default User” could be 'daemon' or 'mailnull'.
the names i have used those are i came across till now. so chances are there you see something diff.

Whole path need to be executable by that user.

to change outgoing IP on your sendmail server.
change following variable.

| sendmail.cf

O ClientPortOptions=Family=inet, Address=192.168.1.22

NOTE : change 192.168.1.22 with your IP.

Check if you want to delete all mail.

mailq
OR
sendmail -bp

Now if you want to delete everything in queue just delete everything inside mqueue folder.
In CentOS it is located at /var/spool/mqueue/

rm -rf /var/spool/mqueue/*

If there is too many files to remove you better use option like xargs with find.

find /var/spool/mqueue -name * -print0 |xargs -0 -r -n100 rm