+1 for O365 or Google Mail for business. You simply can’t create the level of redundancy that these two provide. There are always cheaper options, however that doesn’t mean you’re not trading something for them.
If you roll your own mail server which I have done several times using sendmail, exchange and various other mail servers on a myriad of OS platforms, Solaris, Linux, Windows, etc. The tradeoff here is the amount of time you will spend managing the installation, backing it up, etc. If done correctly, it’s a lot of fiddling with this and that such as backups, configuration changes when something doesn’t work, upgrades to newer versions, etc. If you enjoy these things, by all means, set your own server up. If you go this route, you can get it done for free except for hosting the server.
If you go with a free service, then ads or your email will be the product the company is looking for. I have a couple domains that use zohomail and it does just fine with basic tasks. It is no more difficult to setup than any other mail service where you create DNS records and point mail to their servers.
If you go with O365 or Google Mail for business, you will pay per user, but you get a ton of features that you may or may not want. If you or your entity is a Windows/Office shop, then O365 makes quite a bit of sense. If you tend to use other OS, then Google might make a bit more sense.
As others have said, I wouldn’t try to include batch mail services in your mail platform. It’s just not a service they tend to provide, and you can end up in a bit of hot water if you send out too many emails or they think you’re spamming from their servers. Use a dedicated service that will follow all/most of the rules for unsubscribe links, return email, etc.
Overall the old adage works… Cheap, Fast, Quality - Choose 2