The biggest issue you will likely encounter is having clean IP addresses to send from. If you have that problem solved, the next hurdle will be realizing that Stalwart is more than you would need to manage sending daily emails. If you have the IP addresses, you could just use a script that grabs email addresses to send to for the day, and ship them with a fairly straightforward SMTP daemon (sendmail) installation on a VPS.
Stalwart might be a good use case for your business/employee mail account handling, however.