Currently, my system is forced to use a free mail service called mailjet. Our website gets way more traffic then how much mailjet can offer. Although this may not be possible it would be a life saver.
Please clarify: what is being enforced, and how?
I’ve self-hosted for multiple years, often having switched email services due to one changing their terms, but only due to price comparison.
Are you able to switch to a different email delivery service? Are you able to pay Mailjet to deliver more mail?
Can you give us an idea on number of emails per month?
The easiest way to do this would seem to be external to Discourse – specifically, a tool that accepted mail delivery, then used one of a number of third-party services to relay the mail. I’m not aware of such a tool already existing, but it would seem to be a better solution than trying to build in some sort of mail relay redundancy into Discourse itself.
If self hosted this plugin allows to route email to another email delivery service.