Synapse is a common server because it is the most featureful, but other servers like conduit and dendrite work fine... Bridges are written to interact with the feature set of the server, so other matrix servers may not support all the same things that make the bridge work.
Not sure about the yunohost setup, but there is a lot of documentation around the Ansible deployment and it includes every bridge imaginable, I would start there for a better supported solution.
Upvote for mailcow. It's a classic postfix+dovecot email stack with lots of stuff figured out for you already. I host on a cheap vps that supports SRV records so rDNS passes, and then I still route my outgoing mail through Mailgun for better IP reputation. At my low (normal) mailing levels Mailgun is free.