If you use custom DNS for sending emails, when you send an email in Kolleno to multiple recipients, the system checks each recipient against blacklist records.
If any recipient is marked as blacklisted (for example, due to a bounce or a complaint), the email will not be sent.
What does this mean?
Bounce: This happens when the recipient’s email server rejects the message.
Hard bounce (permanent): The address doesn’t exist, the domain is invalid, or the mailbox is permanently unavailable.
Soft bounce (temporary): The mailbox is full, the server is down, or the message is too large.
⚠️ Hard bounces automatically cause an address to be blacklisted, as continuing to send emails to invalid addresses harms your sending reputation.
Complaint: This happens when a recipient marks a previous email as spam or unwanted. Their email provider notifies our system, and that address is flagged to prevent future emails from being sent.
Both bounces and complaints are recorded to protect your domain reputation and maintain good deliverability with all email providers.
Why does Kolleno block these emails?
Blacklisted recipients cannot receive emails from Kolleno.
This is done to:
Protect your domain reputation (avoiding spam flags).
Improve overall email deliverability.
Prevent repeated sending to invalid or spam-flagged addresses.
How can I resolve this?
Review the list of recipients.
Look for any addresses flagged as blacklisted.Take action on the blacklisted recipients.
If the address was a typo or is outdated → update it.
If the recipient complained (marked as spam) → reach out to them through another channel before re-adding them.
Re-send the email.
Once all recipients are cleared from the blacklist, Kolleno will send your message.
⚠️ Important: Kolleno will only send the email if every recipient in the list is valid and not blacklisted.