Referring to the older thread
Mail MessageGateway is null
I'm facing a similar problem where the emails are sent successfully sometimes and sometimes not.
If I check the logs, when emails are not sent I see this in my error.log
com.day.cq.wcm.foundation.forms.impl.MailServlet The mail service is currently not available! Unable to send form mail.
This translates to me that the object for class com.day.cq.mailer.MailService was null. Please advice.