Firstly welcome to the board, you may want to introduce yourself in the Echo Lounge to get to know a few of us 😉 or maybe not :glare:
Getting to your question
There could be a number of reasons. The email providers may flag the message as Spam for some reason HTML messages get flagged depending on some mail server settings.
I should mention no one here is actually going to just hand out code, you may want to post what you currently have to have it fixed. You may also choose to have a search of board. There are a few questions similar to this that have popped up.