Logging for SMTP emails

I have a site where the outgoing mail settings are set to send via a third party SMTP host.  Emails sent this way are logged in the CMS Outgoing Mail and also in the log of the third party SMTP host.

This is working in general but occassionally an email is sent which is then logged within the CMS but not at the third party SMTP host.

Is there any more verbose logging that we can enable for SwiftMailer to work out why some emails are failing?

Thanks, Paul.