You didn’t quite answer all the questions. Do MailJet logs show an attempt to connect and send email from the IP of the linode? Digital Ocean will show a connection attempt from the Digital Ocean IP in its web UI for email logs.
Looks like something in your network, somewhere, is blocking connections based on
Sent mail to justin@mydomain.tld (30058.0ms)
Job exception: execution expired
You can look at /logs URL in Discourse but you need an admin account to do that.