What’s a 3D doing in this HTML?

It’s an email encoding system called “quoted-printable”, which allows non-ASCII characters to be represented as ASCII for email transportation.

In quoted-printable, any non-standard email octets are represented as an = sign followed by two hex digits representing the octet’s value. Of course, to represent a plain = in email, it needs to be represented using quoted-printable encoding too: 3D are the hex digits corresponding to =‘s ASCII value (61).