Why is WarmUp Blocked for my email sender

title: Why is WarmUp Blocked for my email sender?
description: Understand why warmup gets blocked and how to re-enable it.

Overview

WarmUp can be blocked or disabled for an email sender for several reasons. This document explains each cause, how the block works, and what you can do about it.

1. Bounce-Based Blocks (Auto-detected by the system)

Our system continuously monitors warmup emails for bounces. When a bounce is detected, an AI analyzes the bounce reason and takes protective action to maintain the health of the entire warmup pool.

1.1 Hard Block — 15 Days

A hard block is applied immediately when a critical bounce error is detected. The sender's warmup is disabled and cannot be re-enabled for 15 days.

Common reasons for a hard block:

• Authentication failures — SPF, DKIM, or DMARC checks failed
• Unusual activity / content policy violations — flagged by the ESP
• Warmup service detected — the receiving ESP detected that the email is part of a warmup network
• Gmail unauthenticated sender blocks — Gmail rejected the email because the sender is not properly authenticated
• IP / Domain blacklists — the sending IP or domain is listed on RBLs such as:
• Spamhaus (zen.spamhaus.org)
• Barracuda (b.barracudacentral.org)
• SURBL, SORBS, SpamCop
• Hard blocks from specific providers — e.g., MailChannels (550 5.7.1), Titan/Hostinger suspended accounts, Yahoo (553 5.7.2, 554 5.7.9), Outlook (s3140, s3150, 550 5.7.520, 550 5.7.708)
• 3rd soft bounce — if a sender accumulates 3 soft bounces, the 3rd strike converts into a hard block

1.2 Soft Block — 1 Day (3-Strike System)

A soft block is applied for temporary bounce issues. The sender's warmup is disabled for 1 day.

• 1st soft bounce → 1-day block
• 2nd soft bounce → another 1-day block
• 3rd soft bounce → converts to a hard block (15 days)

Common reasons for a soft block:

• Mailbox full / out of storage space
• Out-of-office / vacation auto-replies
• Insufficient system storage
• Other temporary delivery issues that are not classified as critical or ignorable

Note: DFY (Done-For-You) senders are skipped for soft blocks and will not accumulate strikes.

2. Plan or Subscription Restrictions

Warmup is automatically disabled if the account associated with the sender does not have an active, eligible plan.

What to do: Upgrade or renew your subscription to re-enable warmup.

3. SMTP-Only Senders

Senders with the service type smtponly cannot use warmup by design. These senders do not have IMAP capabilities, which are required for the warmup reply loop (moving emails from spam to inbox, reading replies, etc.).

What to do: If you need warmup, connect the sender with full IMAP/SMTP or OAuth (Gmail / Outlook) credentials instead of SMTP-only.

How to Check If Your Sender Is Blocked

1. Go to Outreach → Email Senders.
2. Find your sender and look at the WarmUp toggle.
3. If warmup is blocked, you will see a warning such as:

   "WarmUp Blocked — Emails being sent to this email address were found to be bouncing off. This has been negatively impacting other email addresses in the WarmUp pool."

4. You can also click "view bounced email" (if available) to see the exact bounce details.

How to Re-enable WarmUp After a Block

Important: If you re-enable warmup while the underlying issue (e.g., blacklist, authentication failure) is not fixed, the sender will likely get blocked again.

Updated on: 03/07/2026