I have a client with an unusual email requirement. They have a form on their site which captures a fairly low level of submissions per day - yet those submissions are *extremely critical*. They understandably do not want to accept anything less than a 100% deliverability rate of their transactional emails.

I have set up the best SMTP solution I can find which integrates the form with a third-party company offering good deliverability, but I am still (very) occasionally seeing forms that are sent, yet not delivered due to the vagaries of the email system (the last one was not being able to find the MX record of our domain, because I assume the DNS server glitched).

What techniques can you suggest to improve deliverability?