sendmail problem

I use the sendmail to send out the mail from unix-base server to the mail server , I found that sometimes when the sendmail service is not start , then if a mail is through the sendmail service , it will be lost , I check that the "/var/spool/mqueue" directory but it is empty and also can't send again by "mailq' , could suggest where is the "dead" mail is stored ?

If i want to find the mail that are lost in the sendmail daemon shutdown period , where can I find it ? thx

mailq
/var/spool/mqueue is empty
Total requests: 0

Often the dead.letter file is in the users home directory.