Email Delivery Tools: SendGrid, Customer.io, Iterable Explained

How Email Delivery Tools Work

You run a TechSpy scan on your domain and suddenly see "SendGrid" or "Iterable" listed under "Email Delivery Tools." You’ve never logged into those services. Is something wrong? It’s more like discovering that your office building has a freight entrance used by a caterer you hired years ago for monthly newsletters. That caterer still has a key, and their logo appears on some invoices—but you’re the one running the business. Email delivery tools are providers that send emails on your behalf. They handle the heavy lifting of getting newsletters, password resets, and onboarding sequences into inboxes at scale. They aren’t security tools; they’re the actual senders. The scan noticed them because, to let them send as your domain, you (or someone in IT) added a small note in your domain’s address book—a DNS record. That note is what TechSpy reads. Tools like SendGrid (now Twilio SendGrid), Customer.io, and Iterable are popular ways to send marketing and transactional email. They manage deliverability, track who opens your messages, and keep you from hitting sending limits on Google Workspace or Microsoft 365.

Real-World Analogy

Think of your domain as a house. Email security settings (SPF, DKIM, DMARC) are like a neighborhood watch list—they say which mail carriers are allowed to drop letters in your mailbox on your behalf. An email delivery tool is the actual carrier who delivers the letters you’ve written. Sometimes they require you to register their uniform (a DNS record) so the watch list knows they’re legit.

Here’s what happens when you send a newsletter through Customer.io.

You compose your campaign and hit send. Instead of your company’s own email server grinding through thousands of messages, Customer.io does the heavy lifting. It uses a one‑time setup you did (like giving it a digital stamp) to mark each message as truly coming from your domain. Then it delivers the email to Gmail, Outlook, and other inboxes. The receiving servers see the stamp, check your domain’s public record (DNS), and if everything matches, they treat the email exactly like you sent it yourself.

There’s no mysterious technology here. Your tool simply acts as a trusted courier, using the credentials you provided once.

Technical Details

▸The stamping works through SPF and DKIM. For SendGrid, you typically add a DKIM CNAME record (like pointing to sendgrid.net) and an SPF entry ().

▸When a receiving server gets an email, it looks up those records to see if SendGrid’s IPs are authorized to send for that domain. If yes, the email passes authentication.

▸Scans detect these tools by searching for known patterns in SPF includes, DKIM CNAMEs, or even email headers (e.g., for SendGrid).

▸Delivery tools are not email security. Security layers like DMARC can be added to tell receivers what to do if authentication fails, but the delivery tool itself just handles the sending.

Why It Matters for Your Business

When everything is set up correctly, your marketing emails land in inboxes instead of spam folders. You can send thousands of onboarding sequences, product updates, or cart‑abandonment reminders without your team manually bcc’ing everyone from Gmail. Delivery tools also give you open rates, click data, and reliable automation—without IT help for every campaign.

If a delivery tool is misconfigured or unauthorized, things break quietly. Customers might miss password‑reset emails. Your sales team’s follow‑ups could vanish. In the worst case, scammers could slip into an old, forgotten vendor account and send phishing emails that look convincingly like you. Your domain reputation suffers, and it can take weeks to rebuild trust with email providers.

This isn’t just an IT concern. Marketing, support, and sales all depend on email reaching real people. When the CEO hears that “customers aren’t getting invoices,” someone needs to know whether the delivery engine still has a valid key to your house.

Common Issues and Warning Signs

You might not hear about a problem until a campaign fails. But the TechSpy scan gives you a head start by surfacing every delivery tool that has a footprint on your domain. If you see a name you don’t recognize, or a tool you canceled months ago, that footprint is worth checking.

Common Issues

Your marketing team reports that welcome emails land in spam, but other messages are fine. (SPF includes the tool, but the DKIM signature is missing or misaligned—email providers see a mismatch.)

You get DMARC reports showing failed deliveries from an IP you don’t own. (An old delivery tool’s SPF include is still in your record, and its servers are trying to send without proper DKIM.)

A TechSpy scan lists SendGrid, but nobody at the company remembers using it. (The SPF include was never removed after a trial or departed vendor, leaving a potential impersonation path.)

Customers complain about not receiving password resets from your app. (The transactional email provider’s DKIM record expired or was deleted, causing authentication failures.)

How to Fix or Improve Your Email Delivery Setup

Start by identifying which tools your teams actually use. Marketing knows if they’re logged into Customer.io; developers know if the app uses SendGrid for transactional mail. Once you have the real list, you can clean up the DNS footprints so only those tools are authorized to send as your domain.

After you clean up, run a free TechSpy scan to confirm that only the delivery tools you actually rely on are visible. A tidy DNS setup means better deliverability, fewer surprises, and one less reason for customers to miss your messages.

1Ask your marketing, support, and engineering leaders: “Which email sending services do we use right now?” Make a list.

2If a tool is legitimate but missing from your DNS, follow that tool’s setup guide to add its required SPF include and DKIM CNAME records. This usually takes 5 minutes and a quick copy‑paste.

3If a tool is on the list but no longer used, log into your DNS management panel (or ask your IT person) and remove every reference to that vendor from your SPF and DKIM records.

4For tools you don’t recognize, treat them as unauthorized until proven otherwise. Remove their SPF includes and DKIM CNAMEs immediately; if the vendor’s account was compromised, your domain becomes an open door.

5If you don’t manage DNS yourself, forward the TechSpy results to whoever does. A simple note like “We seem to have old entries for SendGrid in our domain records—can you review and remove anything not in use?” is all they need.