Why are my warm-up emails not being delivered (Custom SMTP)

If you're seeing a warning like:

“All warm-up emails from your address aren’t being delivered. Check your SMTP setup.”

… this means that MailReach tried to send warm-up emails via your Custom SMTP configuration, but they never reached any recipient inbox — not even spam folders.

What does this mean?

All your warm-up emails are marked as "missing", which typically indicates one of the following:

• Your SMTP provider is silently blocking or dropping emails
• Your SMTP server is misconfigured
• Authentication settings (login/password, ports, encryption) are incorrect
• There are sending limits, IP blocks, or anti-spam filters stopping the delivery

In short: the problem is on your side (SMTP setup) or with your email provider.

MailReach can’t “see” exactly where the problem is — we can only detect that the emails aren’t reaching any inbox.

What you should check

We recommend verifying the following points in your Custom SMTP setup:

SMTP credentials & server details

• Are the hostname, port, and encryption (SSL/TLS) settings correct?
• Are the credentials (username/password) still valid?

Look for bounce messages or SMTP errors

– If your SMTP server returns delivery errors or bounce messages, they usually contain a code or explanation (e.g. “550 5.4.6 Unusual sending activity”).

– These clues can help you or your provider identify exactly what's wrong.

Authentication & sending permissions

• Does the SMTP server allow sending from the address you've connected?
• Is authentication (login) required and correctly configured?

Provider-side limitations

• Are there daily sending limits? Quotas reached?
• Is your account or IP flagged or blocked by the provider?

Technical settings

Are SPF, DKIM, and DMARC records properly set up?

→ You can check this under the “Checks” tab in MailReach.

Common reasons this issue may happen

Here are a few real examples of known cases:

• Provider silently drops messages due to reputation or content

Some providers have spam filters that block delivery without bounce-back.

• Expired or incorrect SMTP credentials

A password change or permission update can silently break the connection.

• You’re using an alias not authorized for sending

Some SMTP servers only allow sending from certain verified addresses.

• You hit a sending limit (daily/hourly)

Once the limit is hit, emails may be silently dropped or delayed.

• Zoho Mail blocks SMTP sending due to “unusual activity”

You might see the error 550 5.4.6 Unusual sending activity detected.

→ Zoho’s usage policy

🧠 We'll keep adding to this list as we discover new causes.

How to test

To confirm that your SMTP is working correctly:

1. Try sending a test email using your SMTP to a mailbox you own and see if this test email lands in the mailbox.
2. Reach out to your provider's support team and ask: "Are emails sent via SMTP from this account being delivered? Are there any restrictions or blocks?"

Still stuck?

👉 Reach out to our support team with:

• The email address affected
• A screenshot of your SMTP settings (with sensitive info blurred if needed)
• Any test results you’ve run

We’ll do our best to point you in the right direction!

Updated on: 15/10/2025