Ubuntu – Postfix unable to create lock file, permission denied

I thought I had my postfix configuration all set up on my Amazon Ubuntu server but I guess not. I'm trying to set up an admin email account for 3 virtually hosted Apache websites. Here's my postfix main.cf file:

myhostname = ip-XX-XXX-XX-XXX.us-west-2.compute.internal
alias_maps = hash:/etc/aliases
alias_database = hash:/etc/aliases
myorigin = /etc/mailname
mydestination = ip-XX-XXX-XX-XXX.us-west-2.compute.internal, localhost.us-west-2.compute.internal, , localhost
relayhost =
mynetworks = 127.0.0.0/8 [::ffff:127.0.0.0]/104 [::1]/128
mailbox_size_limit = 0
recipient_delimiter = +
inet_interfaces = all

virtual_mailbox_domains = example1.com, example2.com, example3.com
virtual_mailbox_base = /var/mail/vhosts
virtual_mailbox_maps = hash:/etc/postfix/vmailbox
virtual_minimum_uid = 100
virtual_uid_maps = static:115
virtual_gid_maps = static:115
virtual_alias_maps = hash:/etc/postfix/virtual

Here's my vmailbox file:

admin@example1.com                  example1.com/admin
admin@example2.com                  example2.com/admin
admin@example3.com                  example3.com/admin
@example1.com                       example1.com/catchall
@example2.com                       example2.com/catchall
@example3.com                       example3.com/catchall

And finally my virtual file:

postmaster@example1.com                 postmaster
postmaster@example2.com                 postmaster
postmaster@example3.com                 postmaster

When I try to send an email to through netcat to my one of my domains, I get:

unable to create lock file /var/mail/vhosts/example1.com/admin.lock: Permission denied

This is despite the fact that I set example1.com group to postfix and also my virtual_uid_maps and virtual_gid_maps are both set to Postfix group id of 115.