Send HTML Email in Gmail or macOS Mail
Gmail and the Mail app on a Mac don’t offer a way to compose an email in HTML within the client. However, you can send an HTML message through either of these two clients by copying and then pasting the HTML code created in another application.
Send HTML Email in Thunderbird
Thunderbird makes it easy to create HTML and then import it into your mail messages.
Tips for Writing in HTML
If you’re going to write your HTML messages in a different editor, there are a few things you should remember so that your messages will work.
Your images need to be hosted on a public website. They don’t have to be on a web page, but they do need to be publicly accessible on the internet. Use the complete URL of the image in the attribute, and test it to make sure your image displays properly before you send the message. Do not use relative paths in links. You should use fully qualified URLs for all links, just as with images. If you are using CSS, embed them in the HTML code using the inline, embedded, or external style sheets.
Email clients may not support advanced features like Ajax, CSS, or HTML5. The simpler you make your messages, the more likely they’ll be viewable by most of your email recipients. Some email clients also make it easier than others to use HTML created in a program or an HTML editor.
An Advantage to Using Thunderbird As Your Email Client
You can connect email accounts such as Gmail, Yahoo Mail, and many others to the Thunderbird email client and use it as your one-stop email app. This way, it is slightly easier to insert HTML into a Thunderbird message, and you can send your outgoing messages through any of your connected email accounts.
Not Everyone Can Read HTML Emails
If you send an HTML email to a person whose email client does not support it, they will get the HTML as plain text. Unless they are a web developer and comfortable with reading HTML, the letter will be difficult to read at best, and little more than unreadable gobbledegook at worst.
