It took a moment to understand why both command line and php mailing processes were taking ages to accomplish.
checking /var/log/mail.log said:
May 17 08:13:01 ip-10-125-9-183 sm-mta[26006]: My unqualified host name (mydomail) unknown; sleeping for retry
I've been checking that the hostname ("cat /etc/hostname") was the same defined into sendmail and hosts.
I've finally changed /etc/hosts like this:
127.0.0.1 localhost mydomain localhost. mydomail.
note the dots added to the domain name.
once updated sendmail started responding immediately
finally, if you don't want your email to get into spam folders automatically, write this form:
https://aws-portal.amazon.com/gp/aws/html-forms-controller/contactus/ec2-email-limit-rdns-request
