how long should the mail () function take to return the response

I am sending emails and it takes me between 8 to 9 seconds to return the answer, I did the following:

• I ran the benchmarks

• modify the content of the mail

The only thing that delays is the answer, how could it be optimized? (taking into account that I have the hosts well configured)

I am using sendmail with a local smtp server of my own domain