What they are describing sounds to me like customer support communications. Customer support orgs will notify customers of changes, alerts on outages, account issues and quite a bit of more sensitive communications that are most commonly either sent via a web portal or through email. Web portals can be hacked and email ... well email is not secure unless all of their customers know how to use GPG which is easy now with Thunderbird but training the world on that would be a monumental task. In my last place about half of the customers used PGP with all their API integrations.
If they are B2B I would suggest GPG via Thunderbird assuming they have IMAPS access to their mail servers. Something like Matrix might also work but they would have to train all their customers how to use it and that might also be a road block and they would still need to implement MFA/2FA to avoid the same account take-overs that affect web portals.