The most useful lesson I learned at art college was that I lacked talent. I was in the same lectures as those with real ability, one or two of whom went on to be quite famous in graphic design. You'd think I'd stand no chance but I completed my course with distinction despite my potential being journeyman at best.
My trick was to listen to my tutor, a chap who was moderately successful without setting anything alight other than his pipe. He gave me pointers, and I had the good sense to follow them. Here are those which are useful for email marketing design.
1/ Copy those marketing emails that others find attractive
I tore out pages of display ads from magazines. My copy of Print in Britain was about half as thick when I'd finished. I used to catalogue the different types so when I had an advert to produce, I just went to my records, picked one that was appropriate, and more or less copied it. Copyright is not that restrictive. Similar is distance enough.
I had more contracts than the really inventive chaps because mine were proven.
2/ Listen to feedback
Email marketing software is harsh but inarguable, just like my tutor. Use it to improve. Remember though it is statistically based. There is still room for objective opinion.
Ask your staff or peers what they think, or better still, what they think will improve it. Do not be general, ask if the colours chosen are the correct ones or if the balance between pictures and text could be better. The response to specific and limited questions is often the most helpful.
3/ You are not a genius
Don't try to be too clever. Leave that to the professionals. All you want to do is produce a design that will engage. Whilst it might be exciting to win awards for innovation, these do not necessarily produce the best returns.
4/ Work to timescales
Designing takes time, even when modifying email marketing templates. You need to perform a task, that of selling. Anything else is a waste of money.
It is pointless trying to set new standards of design. Mind you, don't just go with the flow, but be at the head. See what new styles are catching the imagination and jump on the bandwagon. It worked for me.
6/ Don't go all technical
It is not for nothing that Photoshop is regarded as leader in the field, but it is expensive both to purchase and to learn. Don't go for it just because the professionals use it. You want simple and quick.
7/ Do what you know
Pick a style you are happy with, something that inspires you. Readers are not so much influenced by specific designs but by those which are done well. You'll get better as you go along, and quite quickly, and that is the time to experiment.
To summarise: once I accepted that I was not going to be a creative graphic artist, I concentrated on producing material that worked. Or rather, that sold, which is the same thing. I was making more money than those with much greater skill because I concentrated on the result.