I recommend doing these under three separate but related domains, or at least separate subdomains, and on separate infrastructure. This way, you can allocate a

I recommend doing these under three separate but related domains, or at least separate subdomains, and on separate infrastructure. This way, you can allocate an entire domain (or subdomain) for each function, and if you run into problems with your marketing mail going down the tubes because someone decides to put your IP address ranges on a blacklist, that doesn't automatically nuke your main corporate mail and your transactional mail with your existing customers.