Typically you would only use virtualmailboxdomains etc, if you are hosting more than one domain on the same server. If you are hosting more than one domain use virtual domains, if not then use mydestination. If you are hosting multiple domains the I normally set mydestination to be the FQDN of the server i.e. mailserver.example.tld (as opposed to example.tld for a single domain server). It's probably a good idea to add localhost to your mydestination.
Virtual alias domains do a 1 to 1 mapping of the local part (the bit before the @ symbol in an email address) of the virtual domain to it's target domain but ONLY IF the local part is listed in one of the virtualaliasmaps (you can have none or more maps; separated by commas). These maps (or map) can have various backends; file, hash file, ldap, sql etc.
So in your can you'd probably want to do something like this (a name like <name> can be replaced with anything (don't actually add the angle brackets!));
main.conf
look inside the file called
for the alias mappings
virtualaliasmaps = hash:/etc/postfix/<filename>
/etc/postfix/<filename>
virtual-alias.domain anything
me@allmydomains.tld me@gmail.com
The first entry in the virtualaliasmaps file must have the keyword "virtual-alias.domain" in the first entry. The second entry can be anything as it is disregarded.
After that put all the fully qualified aliases and their destination in the first and second entries respectively. Once you have all the entries entered then run
postmap /etc/postfix/
Whatever you do don't put allmydomains.tld (or any of your other virtualaliasdomains) in either the mydestination or relay_domains config entries; it will break.
