John said:

I think SRS might be what you want here.

It does the mangling and untangling of addresses so you can dkim sign them with your domain.

It does, but unless I have been doing it wrong, it's stopped being enough for certain domains as far as webmail providers like Gmail are concerned.

Using aliases - via Postfix in my case - is how I have done 'send incoming mail to this address to these addresses' for ages, but when Gmail got really fussy for domains that specify they really care about their origin, I have had to switch to the Gmail accounts getting email from the server via POP3.

Ian