In my opinion, high-performance emails always include a subject line, clear and direct information that is coherent with the recipient’s needs. Additionally, they are visually appealing, featuring images and a CTA that motivates the recipient to interact with the sender and achieve the email’s goal.
It must be clear, concise, and coherent
In my opinion, a successful marketing email template should be clear, visually appealing, and focused on a specific objective. The subject line should grab the recipient’s attention, and the content should be concise, relevant, and tailored to the audience’s needs.
I also consider it essential to include a clear call to action (CTA) that tells the reader what to do next, for example: “Buy Now”, “Learn More”, or “Sign Up Here”. Additionally, the design should be easy to read on any device, especially on mobile phones.
Finally, a good email should generate value and trust, avoiding overwhelming the user with too much information. The goal is not just to sell, but to build a relationship with the customer and guide them toward a specific action. 
Straight to the point
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Have a single clear objective: every element — subject line, preview text, body content, images, and CTA — should direct the reader toward the same action.
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Use a simple and responsive template: simple designs render better across mobile devices, desktops, and various email clients, load faster, and keep the focus on the call to action.
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Create a clear visual hierarchy: organize the content so the reader understands the main message first and then knows what to do next.
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Maintain brand consistency: apply consistent colors, typography, images, and tone.
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Use white space and relevant, optimized images: this improves readability and prevents the email from looking cluttered.
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Include a visible, clear CTA that aligns with the objective: it should explain exactly what the next step is and work well on mobile devices.
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Be inclusive and accessible: use good contrast, alt text for images, and easy-to-read content.
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Preview and test before sending: check how the email displays on different devices and email clients.
In my opinion, a good marketing email template is brief, clear, and has a single objective. It features an eye-catching subject line, easy-to-read text, and a visible button that invites clicks.
Example:
Subject: New offer
Text: Check out our special promotion.
Button: View offer
Should definitevely reach the objective as a must. Then include, the right content to achieve this goal.
Through an A/B test, we discovered that our contacts are highly visual and weren’t drawn to emails with too much text. As a result, we incorporated icons along with direct, concise copy into our emails, which led to a significant 53% increase in open rates, an 8.10% conversion rate, and sales of up to $37,000.
We also learned that our target audience preferred receiving emails early Tuesday mornings (we found this out after further analysis).
Additionally, depending on the objective, we included two to three CTAs, always aiming to redirect users to our website where they could complete their purchase.
To help customers feel familiar with the content, we added our website’s menu to our email template. This made them feel like they were browsing our site, even though they were in an email, which helped build trust.
Unfortunately, I wasn’t able to attach evidence… 
The parameters must look complete, eye-catching, and well-structured to ensure order and coherence, without appearing cluttered.