Earlier this year I was asked to set up an email newsletter template. While not necessarily appropriate for the task in hand, someone did recommend using Mail Jet Markup Language (MJML). On a more recent assignment I had to create an campaign email from scratch and as there were no specifications for how this should be done, I decided to give MJML a go.
I have experienced issues either directly or indirectly with organisations that use Amazon's Simple Email Service.
After some investigation my plesk based mail server uses grey listing to cut down on spam emails. One consequence of this is if it does not recognise the mail server it will grey list it, essentially delaying the delivery of the said email. Eventually it will deliver it but chances are any token attached will have long expired.