SendGrid with Rails: ActionMailer SMTP settings

TL;DR: Wire ActionMailer to SendGrid: smtpsettings with the apikey username, plain auth, and STARTTLS on 587. Includes the delivery-method check for each environment. Set the default from to a verified sender: config.actionmailer.defaultoptions = { from: '[YOURVERIFIED_SENDER]' }.

The fix

1. user_name is exactly 'apikey'. Your account email here fails auth.
2. enablestarttlsauto with port 587. Port 465 needs tls: true instead, which is a different setting.
3. In development, keep deliverymethod at :test or use the letteropener pattern so you do not send real mail.
4. A 403-style failure from the API usually means the key lacks Mail Send scope; an SMTP auth failure usually means the username or pasted key is wrong.

When to use this

• This covers exactly what the title says: SendGrid with Rails.
• You are setting this up for the first time, or auditing an existing setup.
• You want the key gotchas in one place before you start.

When not to use this

• You are doing a different workflow with SendGrid; these steps are specific to the title above.
• You need the full reference docs; this is the short path, not the manual.

Compatibility

• Not pinned to a specific version; follows current SendGrid behavior.

Rails ActionMailer SMTP settings plus the official SendGrid SMTP settings. The SendGrid-specific facts are the host, the literal apikey user name, plain authentication, and STARTTLS on 587. Sources: https://github.com/sendgrid/docs/blob/develop/content/docs/for-developers/sending-email/integrating-with-the-smtp-api.md