The free one. But only required until we’ll mount our own server in some data rack, with fixed IP and configuration of reverse DNS (ARP). If needed I’ll get a cheap mail offering from namecheap, or I’ll ask permission to one of my customers with 4 webservers (which I manage) to send mails using their smtp server. I see also WP plugin has options for XOAUTH2 credentials, I suppose you can get them through the developer console. So maybe find a docker based smtp relay with XOAUTH2 authentication.