Your colleague is right and it is a very real risk. Blocklist entries follow the range, not the tenant, and some lists are slow to clear even after a legitimate handover. Before you sign, check the range against the major blocklists yourself, look at passive DNS history if you have access to it, and ask the provider directly what the previous use was. A provider who cannot answer that question is telling you something. We ask about history on everything we take from InterLIR Global
https://interlir.global/ , and it has saved us at least once. Tip: if mail matters to your business, warm the range up gradually rather than dumping full volume on day one. Also set up feedback loops with the major mailbox providers early, they tell you about problems before your customers do.