What is the value of s_wfmail_agent in your apps tier context file?
Note that, The profile option WF_MAIL_WEB_AGENT can be blank, if the profile option APPS_FRAMEWORK_AGENT is pointing to physical host. This is the only condition that it can be blank.
If APPS_FRAMEWORK_AGENT points to load balancing / or a virtual URL then WF_MAIL_WEB_AGENT must set to the physical address.
You need to use the url of that agent as the value of that xml tag. the url of Workflow Mailer Framework Web Agent. Check the context file for the other xml tags and see how these settings are done.. You can do it. This is just a xml update.