Is the account used automatically?

During the first run of letsencrypt-auto, an account was created with the email I specified in my ini file. Now that I see the account ID in /etc/letsencrypt/accounts, is it reasonable to include the ID in my ini file for future runs? What will letsencrypt do when I do not include the ID? Does it lookup the account automatically from /etc/letsencrypt or will it create a new account such that I have two accounts in the acounts/ directory afterwards?

An additional question: What happens when I want to use the same account ID together with another (yet unregistered) email address (e.g. when applying for a cert for another domain)?

I don’t think the “account” is currently really used.

I’m pretty sure the same account will be reused. Email addresses are always for accounts, not for domains. So if you want another email for anothe domain, you’d need another account.