If people don’t receive emails you send using Mail on Mac
If people have trouble receiving email messages you send using the Mail app on your Mac, it’s possible their mail servers are unavailable or have size, storage or other limits. Try these suggestions.
Make sure the person’s email address is correct. Ask the person to send you a message, then reply to it. Now the email address is in the Previous Recipients list in Mail and should be correct.
Some mail servers reject messages that exceed a certain size. If your message includes attachments, use Mail Drop or try resizing image attachments. See Add attachments to emails.
Send yourself a test message. If you receive it, ask people to send themselves a test message to see if the mail server for their email accounts has a problem.
Use Connection Doctor to check your network connection.
Ask your email account provider whether it limits the number of people you can send a message to at one time, as a way to prevent junk mail.
Some mail servers won’t deliver a message to any of the recipients if even one address is invalid. When this happens, Mail alerts you that the message wasn’t sent. Open the message (it’s in your Outbox) and remove or correct invalid addresses, then send the message again.