This is how I do eMail Marketing:

Images should be in line with the ToV of the eMail and brand, giving a clear idea of the subject in hero

Likewise, background, buttons, fonting and palettes should follow style guidelines

Photos could not highlight the product itself, but even something related to the shopping experience

I prefer to put an emotional payoff as the primary text, just below the hero, to engage the customer right from the start

A wise use of hooks and idioms helps selling without shouting (purpose, social proof, usage..)

Codes in primary position, and related CTAs easily visible

In case of promotions by category, I like to show related and/or interesting products

Links to blog articles always help customers to get into the context of the promo

Final check with graphics for consistency, alignment, spacing, and related links

I prefer short claims with meta-references

The visual should already communicate what you want to sell/propose

Little content, but all consistent and neat

Palettes and fonting drawn on the ToV

The brand may be present explicitly (logo, name) or implicitly (colours, lines, claims)