Any web designer who understands accessibility knows the benefits of ALT-tags. As images are not automatically displayed in most email clients, short descriptions of the blocked images give readers a quick overview of what to expect. And after all: ALT-tags help surfers navigate through web pages more easily, so why shouldn’t they work in emails?
Another benefit of ALT-tags is that they help improve the text-to-images ratio in email messages. Which may prove to be your email message’s best defense in the battle against SPAM filters.
Just one more thing: make sure your ALT-tags don’t exceed the width of the images. Some email clients may fail to display them.
Conclusion: Use an ALT-tag for every image in your email message but keep them short and sweet.