The idea isn't actually to use an email alias -- it's to use an email relay.
Sorry if it's confusing (email as a whole is kind of confusing at first glance) -- relaying SMTP through a different server is kind of like using a VPN or a proxy.
Services like ImprovMX and Mailgun can be used for aliasing emails, but they can also just be used to deliver your emails for you (no aliasing to a new address).
The idea is to send all your email (like web traffic through a VPN) to those services.
All my incoming email goes to mailgun, which then forwards it to gmail. It's a so-so solution; it's cheap, extremely simple, and has catchall by default.
The big problem with this, is that gmail considers all messages forwarded to it as coming form the forwarder, not the original sender, and therefore spam harms the reputation of the forwarding domain.
But I don't understand how to configure mailgun to relay messages to gmail instead of forwarding them? Can you explain?
GMail understands ARC ("Authenticated Relay Chain"), a technology that signs a chain of deliveries, which is supposed to allow the right party to be blamed for spam. I don't know if any of these email forwarders respect ARC signature chains, but it would obviously be to their benefit if ARC's attributions were more widely recognised.
Postscript: Neither the Improvmx nor Mailgun sites have easily found information about ARC. It's a relatively complex and not widely used technology; nonetheless, the point I made about its adoption being in their interests stands.
Sorry if it's confusing (email as a whole is kind of confusing at first glance) -- relaying SMTP through a different server is kind of like using a VPN or a proxy.
Services like ImprovMX and Mailgun can be used for aliasing emails, but they can also just be used to deliver your emails for you (no aliasing to a new address).
The idea is to send all your email (like web traffic through a VPN) to those services.