Why does =3D show up in the source code of my HTML emails

What you are seeing is what is called “Quoted Printable Encoding”. It is a very old and common encoding and there shouldn’t be any modern email client that doesn’t understand it. What Quoted Printable Encoding does is it replaces any “High ASCII” character (characters codes greater than 127) into a form that looks like “=87”. Since the equals sign is part of that encoding, the equals sign also has to be encoded into “=3D”. (“3D” is the hexadecimal representation of the equals sign.)

If you believe this is the problem with your HTML, try putting an equals sign anywhere in the rest of the HTML code to make sure that the email is being properly decoded (put in something like “2 + 2 = 4”). When your email client shows the email to you, you should be able to see the equals sign just fine, but if you looked at the source, you would see that it was encoded as well. (In other words, the source should look like “2 + 2 =3D 4”, even though you don’t see the “3D” when the HTML is “rendered”.)