Start a new email with the Compose button. You can format text, pick addresses from suggestions and attach files, up to 25 MB in total per email.
Steps
Desktop
Click the pencil button (Compose) at the top of the window, or press C.
In To, type a name or address and choose a suggestion. Click CC/BCC to add those fields.
Add a Subject and write your message.
To format text, select it and use the bar that appears. Click Formatting at the bottom to keep the bar open.
To attach files, click the paperclip (Attach files) at the top, or drag files onto the window. Images you drag in go into the message; other files are attached.
Click Send, or press ⌘Enter on Mac or Ctrl+Enter on Windows and Linux.
iPhone, iPad & Android
Tap the pencil button (Compose).
In To, type a name or address and choose a suggestion. Tap Cc/Bcc to add those fields.
Add a subject and write your message.
To format text, tap Aa in the toolbar. You get bold, italic, underline, strikethrough, text color, highlight, lists and links.
To attach files, tap the paperclip, then Photo Library or Files.
Tap the send button at the top right.
Good to know
Attachments can add up to 25 MB per email. Newton tells you if you go over.
Suggestions come from people in your mail. The first time you write an email on your phone, Newton asks to use your contacts too. They stay on your phone.
To send from another account or an alias, change the sender at the top of the compose window: use From on desktop, or tap your address on your phone.
Desktop has a few extra tools: block quotes, tables and inline images.