Short answer: No. ColdSend inboxes work only inside ColdSend.
Why the restriction?
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Domain authentication is locked to our infrastructure
When you delegate your domain to ColdSend we generate and host the SPF, DKIM and DMARC records on our name-servers. The private DKIM key never leaves our environment, so no external platform can sign mail on your behalf. -
Inboxes are logically “registered” to our IPs
Each mailbox is tied to a dedicated ColdSend IP that is pre-warmed and monitored for reputation. Exporting the SMTP credentials would let you send from a different IP range, instantly breaking the warm-up chain and putting both your domain and our IP pool at risk. -
We don’t expose SMTP/IMAP credentials
ColdSend uses a closed API; we do not provide host-names, ports, usernames or passwords that third-party tools require. Without those details, Instantly, Smartlead, Reply.io, etc. cannot add the inbox in the first place.
No other email platform allows you to do the same, and for the same reason: none of them (Instantly, Smartlead, Reply.io, etc.) expose SMTP credentials — so the mailboxes stay locked inside the platform where they were created
Bottom line
If you need to split traffic across multiple platforms, register a second sub-domain (e.g. outreach.yourcompany.com) and delegate that sub-domain to the other service. ColdSend will continue to manage the original domain; the other platform can manage its own DNS and reputation independently.