I have seen this minor fact trip a few people when wondering where/why their mail hasn't been delivered.
Prior to APEX 4.x, you would need to run the following SQL to find out details about the background job that manages the Apex mail queue:
select next_date, broken, what from dba_jobs where what like 'wwv_flow_mail%';
Apex 4.x brings us into 21st Century Oracle by using the scheduler module instead:
select job_action, next_run_date, last_run_duration, state from dba_scheduler_jobs where job_name = 'ORACLE_APEX_MAIL_QUEUE';
So this also means you have a number of options to push the mail queue. You can use the API mentioned ealier; call dbms_scheduler.run_job; or log in to the INTERNAL workspace and Manage Instance -> Mail Queue -> "Send all mail"
You might also be interested in the following Apex dictionary views:
And if you're looking at setting up an email server on your swiss laptop (the laptop that does everything - yes, I just invented that term), perhaps start here.
That is all,