Are you getting any of the following error messages:
javax.mail.MessagingException: Exception reading response
java.net.SocketTimeoutException: Read timed out
If so then it most likely is an issue with the SMTP timeout being too low. I would reach out the support team to have them confirm that the root cause of your problem is the timeout. They will also be able to help you increase the timeout if needed as it is a hidden property.