Send out a batch of emails yesterday evening as planned. No Guzzle errors at all. All went well.
Well only one problem, when an email address dosn't exist, and it keeps retrying and retrying, and seems it gets readded to the mail queue, before I know it there are 1000's emails trying to go out to the non existant users email address. This could be part of the Xenforo mail queing settings, perhaps there is a setting to retry 3 times, and if failed remove the email from the queue. Not found it yet. This isn't a new issue it has popped up in the past and likely to be Xenforo core or settings somewhere.
It does seem like the Hot fix has fixed the problem original Guzzle error though.