SMTP Relay replacement - suggestions?

My org is a large public agency and continues to have significant use of SMTP relay due to a range of legacy, still-critical apps and processes in our on-prem environment; these services send email 'streams' on a daily basis via smtp relay (i.e. information/notifications to thousands of employees, emergency communications, a wide range DLs for various purposes, public notifications, etc.). We're in a hybrid Exchange/AD environment and expect to remain so, for the foreseeable future.

Reality: it will be years before these "on-prem SMTP-relay-requiring services" can feasibly be moved to something more current and/or cloud-based.

Can anyone suggest a vendor/solution for SMTP relay from an on-premises environment that they're happy with?