fix typo in ERJavaMail.java

I was checking an issue with setting the smtpPort property and noticed that the default value in line 243 of ERJavaMail.java was looking for er.javamail.smtpPost instead of er.javamail.smtpPort. I couldn't find any other mentions of er.javamail.smtpPost so I'm pretty sure it's just a typo.