We use Nextcloud for this.

Hello.

I run multiple email servers for different companies on multiple servers in different locations all onprem. Some are CTX containers on Proxmox some are just older physical servers.

Do not even try to run it on VPS or on any low reputation IP’s or internet domains.

Use a different internet domain (not subdomain) and IP for your marketing/newsletter/e.t.c. or such that are susceptible for blacklisting, so your business email still get trough.