The main reason some emails reply to the sender domain instead of the designated "Reply-To" address is often related to how automated email systems handle responses.
Here are the primary causes:
Automated Replies Ignore "Reply-To"
Out-of-Office (OOF) or Vacation Messages: When someone sends an automated reply, like an OOF notification, the recipient's mail server typically sends it to the original "From" (sender) address, completely ignoring the "Reply-To" header.
This is standard behavior because the mail server treats the automated message as a system response to the email's originator, not a manual human reply. The "Reply-To" address is generally only honored when a person manually hits the "reply" button.
Authentication and Deliverability Issues
Some email providers, particularly spam filters, view a mismatch between the "From" address and the "Reply-To" address as a potential red flag for spam or phishing, especially if the sending domain isn't properly authenticated with SPF and DKIM records.
While technically it is legitimate to use a different domain in the "Reply-To" header, if the setup is not perfect, it can hurt deliverability, and some mail systems may react by defaulting to the verified "From" domain.
Reply Tracking
Email service providers (ESPs), especially for marketing or automated transactional emails, sometimes use a custom, personalized reply address for tracking purposes (e.g., reply-accountname.activehosted.6.16.28@s15.avl3.acemsrvc.com). This is necessary for their system to capture and read the replies. If reply tracking is disabled in those systems, the address may revert back to the main sending domain.
