A letter that will not send is almost always the sending server refusing the password, a wrong host, or an attachment over the limit. The composer’s status line says which.

Check in this order

1. Is there an account? With no mail account the From line points you at Mail settings, and the Send/Export sheet’s email row falls back to your site, which cannot carry a PDF. Add an account first.
2. App-specific password. Gmail, iCloud, Yahoo and others refuse your normal password from a mail app and want one made in your account’s security settings. Make one and enter it in Settings › Mail for that account.
3. The sending host. Presets fill it in; for another provider, check it matches what the provider publishes for SMTP. Receiving can work while sending fails if only the sending host is wrong.
4. Attachments. The budget is 20 MB in total and the composer says how much you have used. Trim, or send the PDF alone.
5. The From line. With several accounts, the letter leaves from the narrowed inbox’s account, else your pinned default, else the first. Hold the From line to pin the one that works.
6. Network. Fetching can succeed from cache while sending needs a live connection.

What a failed send does

Nothing. A reply that fails is not recorded as sent, a handwritten reply is not filed onto today’s note, and the letter you were answering is untouched. Fix the cause and send again; nothing doubles.

What a successful send does

The letter is recorded in your kept pile as sent, the original is kept forever beside it, and it appears in All our letters with that person.

Sending through your site

If no mail account is set up but a site is connected, the Send/Export sheet’s email row relays through the site. The row says so. It carries the page and the text, not the PDF. If that route fails, the site’s own mail is the thing to check.

Replies threading wrongly

Replies carry the original’s subject and references, so they thread in the recipient’s mailbox. If a reply arrives as a new conversation, the original had no message id to thread on — rare, and not something the app can add.

The Watch

The wrist does not send mail. Marking read is its one verb.

Still failing

Note the exact status text and the provider, and see Report a bug. Do not include your password.