I have now found an article with details on how to test apex_mail from SQL Workshop and when I run following it runs successfully and says "Statement processed." but I do not receive an email.
p_to => 'firstname.lastname@example.org', -- change email address
p_from => 'email@example.com', -- change to your email address
p_body => 'Main email text goes here',
p_subj => 'APEX_MAIL Package - Plain Text message'
I'm not sure how works Apex email queue, I believe the queue is pushed every X minutes.
After having ran your script in SQL workshop, call you call this function:
The query to check the apex queue is
select * from APEX_MAIL_QUEUE;
I hope it helps!