I would prefer to go with the known name and well-established company, instead of LET, for email. For example, Vultr offers $2.5/mo servers from two locations

I would prefer to go with the known name and well-established company, instead of LET, for email. For example, Vultr offers $2.5/mo servers from two locations and they are proven, to be trusted hosting company. In case your IP has a bad reputation (you can test it right after spinning an Instance), you can just shut the server down and create a new one with fresh IP. Problem with both Kimsufi and Online.net is that if you will get a bad reputation IP it is quite hard to get a new one. I have dealt with OVH and a blacklisted IP I have got on a new server and they were expecting me to contact blacklist providers and request whitelisting, which took time and resources.