SMTP · API key as password

How to connect
SendGrid.

Use SendGrid’s SMTP relay with a verified sender and a Mail Send API key.

Connection preview. You can read this guide and save a mailbox for later. Live authorization and warm-up remain off. Do not enter real credentials while the connection form is disabled.

Before you start

  • A SendGrid account and verified sender identity.
  • An API key with Mail Send permission.
  • A sender address authorized by your SendGrid account.

Step-by-step setup

  1. Verify your sender

    Complete sender verification in SendGrid for the address or domain you intend to warm. Adding an address in MailboxWarm does not verify it with SendGrid.

  2. Create a sending API key

    Create a dedicated API key with Mail Send permission. For SMTP, the username is literally apikey; the API key itself is the password.

  3. Choose Other provider in MailboxWarm

    Open Mailboxes → Add mailbox, choose Other provider, and enter your sending email address. Named SMTP services all use this same Other provider connection option.

  4. Enter the connection details

    When connections are enabled, enter the SMTP values listed below. Choose Test & connect. MailboxWarm checks authentication and the encrypted connection without sending a message.

Connection settings

Match these values to the fields in MailboxWarm. Keep passwords and keys private.

Connection option
Other provider
SMTP host
smtp.sendgrid.net
Encryption / port
STARTTLS / 587
SMTP username
apikey
SMTP password
Your SendGrid API key

SMTP sends; it does not read your inbox. This connection does not include IMAP access to your mailbox. Placement and rescue activity concern the controlled seed inboxes receiving warm-up messages. Provider-specific compatibility still needs live validation.

Check the result

In today’s preview, expect Mailbox saved for later. When connections are enabled, a successful check shows Mailbox connected, with warming still paused. A failed connection check is not proof of delivery or a reason to add the same mailbox again.

Once live warming is available, choose the mailbox’s plan and schedule before starting. Connecting alone does not begin warm-up, and your provider’s sending limits still apply.

Open setup preview

Troubleshooting

Authentication fails

Enter the literal word apikey as the username. Check the key’s Mail Send permission and remove accidental spaces from the password.

The sender is rejected

Confirm the address belongs to a verified sender identity and is permitted by your SendGrid account.

Provider resources

Provider menus and account policies can change. Use their official documentation for the latest requirements.

Settings reviewed September 12, 2026 · Documentation review, not a live connection test.