Order of loading and checking userdb on Ubuntu

When using the lmtp service for local delivery, I'd like to change the
user lookup to only use the user portion of the email address.

I think I do that in the 20-lmtp.conf file, but not exactly sure how.

From what I've found on the web, I just need to change the
auth_username_format=%n.

I feel confident that I'm not doing it right.

older posts implied that the order things are loaded is an issue, but
not sure if that is the case anymore.

My goal is to set username_format for only lmtp.

Thanks for any help.