The 7.x API Gateway utilizes exim which out of the box is causing a failure, instructions to correct the fault, as well as sample usage are provided in this document.
[root@SSG71]# echo "This is the main body of the mail" | mail -s "Subject of the Email" user@Somemail.com
In the default out of the box all the mail messages will go to "frozen"
If you run o/s command you will see the frozen or email messages status.
To remove the frozen messages run,
[root@SSG71]# exiqgrep -zi|xargs exim -Mrm
But when reviewing the exim logs you will find that exim is getting an exception thus why these were going frozen,
more /var/log/exim/main.log or tail /var/log/exim/main.log
You will find error,
smtp transport process returned non-zero status 0x000b