As with most mistakes, the most important response is to ensure you do not make it again. It’s not a simple case of ‘Oh, I’ll remember that’. You’ve already missed the error once. What’s to say you would not make an identical mistake again by just not seeing it?
For this article we’ll assume you have checked the content complies with all the relevant laws in all the countries you are sending it to.
Thankfully, there are well-practised systems you can use to ensure you keep the number of errors you make down to the bare minimum, and that’s about as low as you can get. Every business, or rather every successful business, has checks established to ensure everything is as intended before the point of no return is reached. You’re not in the middle of the Atlantic at 40,000 feet, but that Send button is pretty final.
A graphic artist might well spend hours looking at a page spread before they feel it is perfect. They’ve probably seen it in their sleep. In pre-computer days, many would hold a mirror up to the pages to 
check if the layout was balanced. It’s literally looking at it a different way. It’s easier nowadays in email marketing because all we have to do is to reverse it on a screen to ensure its as we want it.
In a similar vein, the person who has completed an email marketing template would have looked at it so frequently that they can be blind to errors. It’s risking one’s self-image to pass your work to another person only to have them to point out a literal almost instantly; but it has to be done.
With mirrors and smug colleagues now in the past, we move on to less esoteric errors, ones which are easy enough to check. ‘There’s nothing worse than . . .’ is a phrase used by people when describing a minor irritation, but when reading a marketing email, there’s nothing worse than a broken link. Your subscribers trust you with their money. If they click on a link expecting to be taken to a landing page but end up with a 404 on the screen, you’ve let them down. They’ll probably remember the error for ages.
Click on every link. That means ensuring it goes where you want it to and, more importantly, where your subscribers want to go. There’s more to this than just clicking on something that is underlined. You have to ensure that you’ve not inadvertently put a hotlink onto, for instance, an image, which means you have to click on every one. It includes social media buttons of course.
Check the copy for spelling and grammatical errors. If you’ve edited the copy whilst it’s been on the email marketing template, copy and paste it into a word processor, but don’t depend on its spellchecker alone. It’s boring I know. It’s also essential.
Check the email on any device your subscribers might use. Look for distortions, poor colour, poor formatting and likely confusion. Check and check again.
