Apple started offering a service where you can use up to 3 email addresses per domain and treat them as real email accounts, meaning that you not only receive email as email@example.com but you can respond using that address. This is what I’m using for my own firstname.lastname@example.org email account. The problem with this is that it can only be extended to people in your family group. There is not a “catch-all” option, meaning that anything to @y.z gets sent to a person. Instead anything that doesn’t match the specific email addresses is dropped on the floor.
There’s another method called forwarding, where you receive email to email@example.com but you respond to it from your Gmail or Outlook or whatever email provider you’re using.
Before that, I used a company called ImprovMX. For free, they provide up to 25 aliases for a domain. This would let us have say firstname.lastname@example.org going to Cathy but the reply would be from her regular email address.
They offer a premium service but it’s not clear to me that it would let somebody reply with a @troop351.org email address. I’ve asked for clarification.
List of cheap email providers, where cheap is defined as around $1/user/month