Honestly, free domains on Instantly sound like a quick win until you hit the deliverability wall. I've tested a few of those free TLDs (.tk, .ml, .cf) as throwaway domains for very low-volume warmup, but the moment you scale past 100 sends a day, most inboxes flag them within 48 hours. Microsoft and Google both treat free TLDs as high-risk because they're abused so heavily by spammers - even with SPF, DKIM, and DMARC set correctly.
What I've seen work better is using cheap, real TLDs from namesilo or porkbun - .xyz or .click can be had for under a pound a year, and they carry way more reputation. Pair that with a proper mailbox provider like MXRoute or Google Workspace, warm up gradually over two weeks, and you'll get a usable domain for under £20 total. Free domains just aren't worth the headache when the entire outreach sequence gets burnt because one domain tanks the sending reputation.
If you're dead set on testing free ones, limit them to strictly cold sub-30-volume campaigns and rotate them weekly. But honestly, the setup time wasted resetting SPF records every time a free registrar suspends the domain cancels out any savings.
