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.
What a warmup block means
A bounce-based block is about emails coming in to your mailbox, not emails going out of it.
Other mailboxes in the warmup pool send warmup emails to your sender. When those emails bounce (get rejected by your mail server), every pool mailbox that writes to you takes a reputation hit. To protect the pool, SalesBlink stops warmup for your sender.
This is why your own SPF, DKIM, and DMARC being set up correctly doesn't prevent a block. Those records help your outgoing emails. A block happens because your mail server, mailbox, or domain is rejecting emails sent to you.
You will see one of these messages on the sender:
- Due to suspicious activity or bounces, this email can not be warmed up.
- 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.
Note: A warmup block does not stop outreach. The sender can still send sequence emails while warmup is blocked.
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 (30 days on a repeat block)
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.
If the same sender gets hard blocked again after a previous block, the block lasts 30 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.
| Reason | Description |
|---|---|
| Free or Cancelled plan | Warmup is not available on free or cancelled subscriptions. |
| Trial expired | If the account is on a Trial plan and more than 14 days have passed since the trial started, warmup is disabled. |
| Subscription expired | For paid plans, if the subscription period has ended, warmup is disabled. |
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
- Go to Outreach → Email Senders.
- Find your sender and look at the Warmup column.
- 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."
- In the WarmUp Blocked popup that opens when you click the red ✕ in the Warmup column, click view bounced email (if available) to see the exact bounce message. The bounce message tells you why your mail server rejected the warmup email.
Common bounce messages
Match the text in the bounced email to the table below. In almost every case, the fix is on your mail server, domain, or email provider, not in SalesBlink.
| Bounce message contains | What it means | Who fixes it |
|---|---|---|
550 High probability of spam |
Your mail server's inbound spam filter rejected the warmup email. | You or your email/server provider: relax inbound spam filtering for the mailbox. |
all hosts ... have been failing for a long time |
The mail servers for your domain could not be reached for a long time. | Your email or hosting provider. |
5.4.4 no mail hosts |
No mail server (MX record) was found for your domain. | You, in your DNS: add correct MX records. See Domain Technical Setup. |
reverse host lookup check failed |
Your mail server rejected the email because of a reverse DNS check on the sending server. | Your email or server provider: change the inbound policy so warmup emails are accepted. |
550 mailbox unavailable – invalid DNS MX |
Your domain's MX records are invalid or point to the wrong server. | You, in your DNS: fix the MX records. |
550 5.7.708 |
Microsoft is rejecting email for the tenant, common with Outlook trial tenants. | You and Microsoft. See Outlook Tenant Error 550 5.7.708 Access Denied. |
Forwarding rejections (for example JFE040003) |
Your mailbox forwards incoming email to another address, and that address rejected it. | You: remove the forwarding rule or fix the destination mailbox. |
Note: Self-hosted, cPanel, and other custom mail servers are the most common cause of warmup blocks. SalesBlink support can't debug or configure your mail server. For warmup that runs reliably, we recommend Google Workspace or Microsoft 365.
How to Re-enable WarmUp After a Block
| Block Type | When Can You Re-enable? |
|---|---|
| Soft Block | After 1 day from the block timestamp |
| Hard Block | After 15 days from the block timestamp |
| Repeat Hard Block | After 30 days from the block timestamp |
| Plan/Subscription | After upgrading or renewing the subscription |
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, and a repeat block lasts 30 days.
Warmup can't be turned back on before the block period ends. This also applies to the API: an update_sender call with warmup_enabled: true returns an HTTP 400 error while the sender is blocked.
Follow Best Practices to Avoid Blocks
Most blocks are preventable. Follow our best practice guides and use recommended senders to keep your warmup running smoothly:
- Best Practices for Email Sender or Cold Email Mailbox — includes the Recommended Senders list for warmup-safe providers.
- Email Deliverability (Best Practices) — authentication (SPF, DKIM, DMARC), sending limits, and domain hygiene.
- Best Practices for Cold Email — recommended email providers and general cold outreach guidance.
Sticking to recommended senders and these best practices significantly reduces the chance of bounces, blacklists, and warmup blocks.
Frequently Asked Questions
My mail server is receiving emails, so why is WarmUp still blocked?
Even if your mail server appears to be receiving emails normally, it may still be rejecting (bouncing) the warmup emails at delivery time. This typically happens when a custom mail server's inbound spam filter refuses the message after the message data is sent (for example, with a 550 High probability of spam error). A bounce like this is enough to trigger a block:
A message that you sent could not be delivered to one or more of its
recipients. This is a permanent error. The following address(es) failed:
john@example-company.com
host mail.example-provider.com [203.0.113.10]
SMTP error from remote mail server after end of data:
550 High probability of spam
Action: failed
Status: 5.0.0
Diagnostic-Code: smtp; 550 High probability of spam
This generally happens with custom mail servers. To fix it:
- Update your custom mail server's configuration so that no incoming emails are bounced (for example, disable or relax aggressive inbound spam filtering for your mailbox), or
- Reach out to your custom email service provider or server provider and ask them to ensure that no incoming emails to your mailbox are bounced.
Once incoming emails stop bouncing, re-enable warmup as described in How to Re-enable WarmUp After a Block above.
My SPF, DKIM, and DMARC are all correct. Why was my sender blocked?
A block is caused by emails sent to your mailbox bouncing, not by your outgoing setup. Your authentication records can be perfect while your mail server still rejects incoming warmup emails. Click the red ✕ in the Warmup column, then view bounced email in the popup, to see the rejection and use the common bounce messages table to find the fix.
Can I still send sequence emails while warmup is blocked?
Yes. The block only stops warmup. Your sender can keep sending outreach emails.
How long does a block last?
A hard block lasts 15 days. If the same sender is hard blocked again, the block lasts 30 days. A soft block lasts 1 day, and the 3rd soft block turns into a hard block.
Why do I get an HTTP 400 error when I enable warmup through the API?
The sender is still blocked. update_sender with warmup_enabled: true returns HTTP 400 until the block period ends.