Usually to get things working/running for cost-free, you yourself will need to do some own learning+configurations.
By the way, you can load "VirtualBox" from Oracle. Its a hypervisor software, that is : it allows to create virtual computer machine (aka: guest machine, aka: VM) inside a physical/real computer machine (aka: host machine).
you can create a VM inside virtualbox software that has x86 / x64 based virtual-hardware to load Windows-10 / Win10.
assign 3GB RAM for the windows VM, assign 25GB virtual-HDD space, assign atleast 1 or 2 cpu (thread) to it, assign a virtual cd/dvd drive, etc etc.
get Windows-10 ISO installer file only from microsoft website, attach the ISO file in the virtual cd/dvd drive, boot from cd/dvd drive, you can load winodws-10 in it, skip registration during installation to load in Trial mode, you can register later (within 30 days, there are ways to extend it upto 60 or 90 days max, so register before that).
Spend time to fine tune the Win10, by disabling all the extra graphics / feature display related options, disable all the indexing related features/options, disable other unnecessary services/processes/functions, etc ... so that it can consume less resource inside the virtual-machine (VM), then it will run better or smoother. (Thre are free scripts that can remove OS's bloatware).
in virtual-windows10 inside virtualbox, you can run windows-10 compatible apps, & load windows-based SERVER apps that are cost-FREE.
you will need to configure virtualbox configurations further to expose network-ports from (win10) guest , to (macOS) host machine , by using specific virtualbox related commands, so pls see virtualbox website's documentations, commands can be specified in some config file as well.
so above solution, can be used as your own IMAP+SMTP email-gateway , existing inside your own mac computer.
And this can be used from your apple Mail app or from Thunderbird app, etc, that are running in your macOS in your host machine (mac / macbook / iMac computer).
Load multiple free SERVER/GATEWAY(s) inside virtual-Windows10, use different network ports for each,
then use them from macOS email-client apps.