These are my suggestions after decades of writing when, for no apparent reason, I find it difficult to start.
1/ Avoid looming deadlines
Give yourself as much time as possible to create the copy. If you won’t know the precise requirements of a marketing email until nearer sending time, you can, at the very least, ghost well before.
2/ Write something
Whilst this might seem an impossible task given the specific problem, try writing a list, instructions, a description of the product. In fact, writing something normally gets your mind into the frame. Consider using an online forum.
3/ Have a break
For a few minutes, or even an hour, do something else. Attack your in-tray, walk through the office, or better still make your way to Starbucks. When you return to the desk, you might well have cracked the problem.
Populate the page with what you want to cover. Use brief headings, move them about and then fill in the gaps.
5/ Don't be critical
Keep writing, even if you know you will discard most of it. Eventually you will get going and when it is time to edit, you will be able to improve the poor material and build on the good.
6/ Read something similar
Search for articles on your subject matter. If your marketing email is about the latest design of winch, then read about winches. You might well find something you can use to 'hang' the article around.
7/ Read something else
Give your mind a rest. Read the newspaper, go online, but pick a subject that has nothing to do with what you need to write.
8/ Promise yourself a treat
Keep telling yourself that when it is finished you'll have a biscuit, break or coffee. It works for me, even when I don't have the block.
If you are worried that you don't know enough, then research before you start writing or ask someone.
10/ Go pro
I'll put a plea in here. Getting a professional to write your copy might well be cost effective if you spend most of your time in coffee shops, walking around or reading the newspapers.