This is probably the only email Drag And Drop builder that is free and produces compatible email HTML code.
What I usually do is start with a template and delete everything, then add in contents.
It allows you to customize every single detail of the email.
When you download it, it will (on the free version) add a watermark at the bottom of the email, which you can easily remove by editing the resulting HTML.
Then, minify the resulting HTML and use it as your mail template, for example, in Toolset Forms.
An account is required to put your hands on the source code, and when you add images, make sure to swap the URLs it produces in the
src tags with URLs of images stored on a server you own (you can also store it with them, but I guess you’d not want that )
Direct Mail for Mac
This tool instead is a Mac-only local App, it is mostly free too but adds a watermark you cannot remove. However, it is quite powerful if you want to write emails locally with your custom template. It is however mostly used for “bulk” mails, for example, if you do marketing or promotions this might be useful.