These instructions show how to add an SMTP server to an existing Mail account, but you can also add these settings at the account creation stage.
Go to Mail -> Preferences (⌘,). On the account you want to edit, click the dropdown that says Outgoing Mail Server (SMTP):, and select Edit SMTP Server List….
Next click the + under the list of SMTP servers, and add a new one.
Then enter the server details:
- Server Name: smtp.sendgrid.net
- Description: SendGrid
- Ports: Use default ports
- Encryption:Use Secure Sockets Layer (SSL)
- Authentication: Password
- User Name: SendGrid account username
- Password: SendGrid account password
When writing a message, there is a dropdown beneath the Subject field, where you can define the outgoing server you wish to use. This way, you can send individual messages to your regular SMTP, and the messages you need to send through SendGrid can go through us. Please note that when you select this, it will be remembered until you change the selection.
Unfortunately, there is not an easy way to add SMTP API headers to a message in Mail.
Mail sometimes attaches images in a way that is incompatible with other email readers, but this is correctable.