I use Thunderbird with multiple GMail.com, Outlook.com, and AOL.com accounts with no ads. It's always been free. Only use Outlook at work. Even-though, Microsoft licensing allows us the latest MS Office suite for less than $20. on personal systems.
Just note that domain name based email may afford ad-free emails. But its accounts and contents no more secure than using a free email provider? Not unless you setup, run, maintain your own mail servers. You're relying on the shared (?) hosting provider to do so? Plus, their server admin has full access to all your accounts and contents. Nothing is really private.