My apologies, I mistakenly believed you had a support plan with FusionAuth (let me know if I am mistaken).
Regarding your question --
Have you had a chance to review the troubleshooting guide that we publish for email?
Lastly, a good tip is to remove FusionAuth completely from the email equation by using SWAKS. SWAKS will complete an email exchange with just your email provider credentials (thereby removing FusionAuth from the equation). If SWAKS fails, then would offer good troubleshooting information.