Limitation of using SMTP when emails are not sending

If you're using an SMTP provider to send emails and they're not delivering most of the time, the reason is usually that the sender email doesn't match the SMTP email you've configured. This article covers how to spot that mismatch, how to read the error the platform shows you, and how to verify a sender identity for the most common SMTP providers.

The most common cause: a sender email mismatch

If your SMTP provider is connected but most emails still aren't delivering, check whether the sender email matches the SMTP email you've configured. This is the single most common reason SMTP-sent emails fail.

Checking the error in a conversation

Once your SMTP provider is set up, if you get an error when you send a test email in the Conversation, you can click the ⚠️ (red triangle) icon to view more details about the error.

The error shown in the conversation is fetched directly from your SMTP provider. If you're already sending from the SMTP-integrated email address and still seeing the error, open a support ticket with your SMTP provider so they can share the delivery status of the email.

Verifying your sender by provider

Each SMTP provider handles sender verification a little differently. Use the guide below for the provider you're connected to.

docs.sendgrid.com/ui/sending-email/senders

Frequently Asked Questions

Before you begin

“Limitation of using SMTP when emails are not sending” is part of the Marketing → Marketing Settings area in Adaptive AI. Configure defaults, tracking, sender details, and connected accounts. Email configuration affects deliverability, reputation, consent, and reporting. Test the exact sender, content, links, and recipient state before sending to a real audience. The objective is not merely to save a setting; it is to confirm the result works for the staff member or customer who will depend on it.

• Confirm the sending domain or provider is connected, authenticated, and approved for the From address you plan to use.
• Verify the audience has appropriate consent and exclude unsubscribed, bounced, suppressed, or otherwise restricted recipients.
• Prepare a small internal test segment with multiple mailbox providers and complete contact fields for merge-value testing.
• Review the subject, preview text, sender name, reply-to address, links, fallback values, and required business identification.
• Check whether the email is part of a campaign, workflow, course, invoice, or system notification so you test the correct sending path.

Verify the result

After completing the instructions in this guide, use a controlled test before treating the setup as finished. A saved screen or success message confirms configuration, but only an end-to-end test confirms the business outcome.

1. 1Send to the internal test segment and confirm each mailbox receives the message from the intended sender without broken merge values.
2. 2Open the message on desktop and mobile, then test every link, image, button, reply path, and unsubscribe control.
3. 3Review delivery logs for accepted, delivered, bounced, blocked, complained, opened, and clicked events where available.
4. 4Confirm replies return to the correct inbox and that the related contact conversation or workflow updates as intended.
5. 5Only expand the audience after the test content, recipient filters, timing, and tracking behavior are all correct.

Troubleshooting checklist

If the result differs from what you expected, preserve the current state and isolate one variable at a time. Start with the checks below before deleting the configuration or rebuilding it from scratch.

• Check domain authentication, provider status, From-address alignment, and sending limits when messages are rejected or never leave the queue.
• Review suppression, bounce, complaint, unsubscribe, and invalid-domain status for a recipient-specific delivery failure.
• Use the email log and provider response rather than repeated test sends to identify the exact rejection or deferral reason.
• Confirm merge fields have fallback values and that the test contact contains the data expected by the template.
• Compare campaign or workflow scheduling, account time zone, smart-send rules, and quiet hours when delivery is delayed.

Best practices

• Send relevant content to well-defined segments and avoid sudden volume increases that can damage domain reputation.
• Use one recognizable sender identity, working reply path, accurate subject, and clear unsubscribe option in marketing messages.
• Remove chronically invalid addresses and investigate complaint or bounce spikes before sending the next campaign.
• Keep templates focused, accessible, mobile-readable, and free of unnecessary tracking or image weight.
• Record major sender, domain, audience, and automation changes so deliverability shifts can be traced to a specific event.