That being said I also setup email@example.com which has never existed. This works internally i.e. domains on the same server, but externally firstname.lastname@example.org isn't working at least externally with say protonmail.( fails to deliver)
I believe this is more a DNS issue for c.com for me than it being unsupported as b.com works both externally and internally, and if setup in webmail for an alias identiy it replies as if from email@example.com even externally.
By webmail I also don't mean roundcube, but a version of roundloop called snappymail added into nextcloud connected to dovcot.
Don't overlook the lack of DKIM for b,com/c.com as that can and will possibly cause issues with deliverabilty.
@Adam is very knowledgeable, I am just sharing what I have tested working for me.