10.1 Outbound Mail Servers (Campaign Broadcasting)

What This Feature Does #

An Outbound Mail Server is the SMTP configuration used to deliver bulk email campaigns. When you create an email campaign, you choose which server sends it.

When to Use #

• Use when setting up email broadcasting capability for campaigns.
• Do not use this for receiving replies — that requires a Reply-To Mailbox.
• Do not use this for system emails (password resets, invitations) — those use a separate platform-managed server.

Prerequisites #

• CompanyAdmin role.
• Pro plan.
• Valid SMTP credentials from your email provider.

10.1.1 Supported Providers #

Which SMTP provider should I choose? #

When should I use this vs alternatives? #

• Choose Brevo or SendGrid if you want the fastest path to a working sender.
• Choose Amazon SES if cost efficiency matters more than setup simplicity.
• Choose Mailgun if your team is comfortable with more technical configuration and diagnostics.
• Choose Postmark if you value delivery consistency and sender reputation over marketing extras.

Generic SMTP (any provider)

Required fields:

• SMTP Host (e.g., smtp.yourprovider.com)
• SMTP Port (typically 587 for TLS, 465 for SSL, 25 for unencrypted)
• Username
• Password
• SSL/TLS setting
• From Email Address
• From Display Name

SendGrid

Website: https://sendgrid.com Required:

• API Key (from SendGrid API Keys settings)
• From Email (must be a verified sender identity)
• From Name
• API endpoint: smtp.sendgrid.net, Port 587, Username apikey, Password = your API key

Common errors:

• 535 5.7.1 Authentication failed — API key is incorrect or not enabled for SMTP
• 550 5.1.1 The email account doesn't exist — From address not a verified sender
• 403 Forbidden — Account suspended or sender not verified

Brevo (formerly Sendinblue)

Website: https://app.brevo.com Required:

• SMTP Key (from SMTP & API → SMTP settings)
• From Email (must be a verified sender domain)
• SMTP Host: smtp-relay.brevo.com, Port 587

Common errors:

• Authentication credentials invalid — SMTP key not the API key; use the SMTP-specific key
• Sender not authorized — verify sender email domain in Brevo

Mailchimp Transactional (Mandrill)

Website: https://mandrillapp.com Required:

• Mandrill API key
• SMTP Host: smtp.mandrillapp.com, Port 587
• From Email (must be a verified sending domain)

Common errors:

• 535 Invalid API key — API key is wrong or revoked
• 550 Unauthenticated senders not allowed — sender domain not verified

Amazon SES

Website: https://aws.amazon.com/ses/ Required:

• SMTP Credentials (generated in SES Console → SMTP Settings, NOT the AWS IAM key)
• SMTP Host: varies by region (e.g., email-smtp.us-east-1.amazonaws.com)
• Port: 587 (TLS) or 465 (SSL)
• From Email (verified in SES)

Common errors:

• 535 Authentication failed — using IAM credentials instead of SES-specific SMTP credentials
• 454 Throttling failure — sending above your SES sending rate limit
• 550 Message rejected: Email address is not verified — From address not verified in SES
• Account still in sandbox — SES sandbox only allows sending to verified addresses; request production access

To request production access: SES Console → Account Dashboard → Request Production Access.

Mailgun

Website: https://app.mailgun.com Required:

• SMTP Login: postmaster@yourdomain.mailgun.org (or your domain)
• SMTP Password: your Mailgun SMTP password
• SMTP Host: smtp.mailgun.org, Port 587
• From Email (must match your Mailgun domain)

Common errors:

• 535 Authentication failed — wrong SMTP login or password; check Mailgun dashboard
• 550 Domain not allowed — sending domain not verified in Mailgun

Postmark

Website: https://postmarkapp.com Required:

• Server API Token (acts as SMTP password)
• SMTP Host: smtp.postmarkapp.com, Port 587
• Username: your Server API Token (same as password field)
• From Email (must be a verified sender signature)

Common errors:

• 401 Unauthorized — invalid API token
• 422 Unprocessable Entity — sender signature not verified; add From email as a Sender Signature in Postmark

10.1.2 Add an Outbound Mail Server #

1. In the sidebar, click Email → Mail Servers.
2. Click Add Outbound Mail Server.
3. Fill in:
   • Server Name (internal label)
   • Provider (or Generic SMTP)
   • SMTP Host, Port, Username, Password
   • From Email and From Display Name
   • SSL/TLS setting
4. Click Save.

10.1.3 Validate a Mail Server #

After saving, always validate before using in a campaign:

1. On the Outbound Mail Servers list, click Validate on the server row.
2. A test email is sent to your account’s email address.
3. Check your inbox for the test message.

Messages:

• Validation email sent successfully. — server is working.
• Validation failed: {details} — review the error details and check credentials.
• Your account does not have a valid email address for validation. — update your profile email first.

10.1.4 Linked Mailbox Warning Indicators #

Outbound Mail Servers can show a linked mailbox warning count when one or more Reply-To Mailboxes that use related email infrastructure are in a warning or paused sync state.

Use this indicator as an investigation shortcut:

• Open the affected Outbound Mail Server row to see whether linked mailbox warnings are present.
• Then open Reply-To Mailboxes to find the specific mailbox showing the warning badge or paused sync badge.
• The server itself may still validate successfully for campaigns even when a reply mailbox tied to the same infrastructure needs attention.

10.1.5 SMTP Error Code Reference #

Errors, Messages, and Common Fixes #

What happens if this fails during a campaign? #

• New emails stop sending from that broadcast attempt.
• Some recipients may already have received the message before the failure occurred.
• The campaign remains incomplete until you fix the SMTP issue or choose a working server.
• Retry only after validating the sender again.

Best Practices #

• Configure SPF, DKIM, and DMARC records on your sending domain to improve deliverability and avoid spam filters.
• Use a subdomain for sending (e.g., mail.yourcompany.com) to protect your main domain reputation.
• Warm up new sending IPs gradually — start with small batches before large campaigns.
• Monitor bounce and spam complaint rates; high rates can suspend your provider account.