Here is a description of chroot:
It was the first result of my Google search.
Since "secure" is relative, there is no answer to wether it is "secure." It is more secure than running it w/o chroot, tough.
I googled "sendmail + chroot" and got the following result:
It seems to be a guide for it.
Also, the following link says how to chroot most services.
I also found it by using Google.