Good email design is mostly restraint. Your email will be read on a small screen, in a hurry, in apps that each render it slightly differently. Design for that and it looks good everywhere.
One column, one goal
Stack content in a single column: heading, a short paragraph, an image, a button. Multi-column layouts break on phones. Decide the one thing you want people to do and make that the most visible button.
Type that reads on a phone
- Body text at 16px or larger; headings clearly bigger.
- Email-safe fonts (Arial, Georgia, Verdana and similar) display the same everywhere; custom web fonts often fall back anyway.
- Dark text on a light background is the most readable combination.
Images: helpful, never essential
Some inboxes block images until the reader allows them. Never put important words only inside an image, always add alt text that describes it, and keep the file small so the email loads fast on mobile data.
Plan for dark mode
Some email apps (Gmail on iPhone, Outlook) invert colours in dark mode; others keep your colours. Avoid text that's only readable on one exact background, and check your email in both modes before sending.
404 Emails' preview has a Light inbox / Dark inbox switch, so you can see the inverted version before you send.
Make the button obvious
A real button with a short action label ("Book a table", "See the collection") beats a text link. Give it enough padding to tap with a thumb and a colour that stands out from the rest of the email.
Keep your brand consistent
Use the same logo, colours and fonts every time, so people recognise you before they read a word. A Brand Kit saves them once and applies them to every new email.