Exchange 2019 On-prem Distribution List issue

We migrated from 2010 to 2019 earlier this year and have an issue with Distribution Lists. When our external contacts post to a DL, Exchange isn't rewriting the headers correctly and anti-spam/anti-spoofing solutions at some other external contacts' organizations are rejecting the messages. MS Support is aware of this and told me that they note it only as an 'internal bug' and will not correct the problem. Rules to rewrite the header "from" field to that of the dist. list haven't worked. Has anyone else seen this or have a possible workaround? I'm being asked to try to find other listserv software to bolt onto Exchange(!). Thanks in advance.