In order to be able to send mails, you have to define at least one account subsection in your config: There needs to be a section "accounts", and each subsection, indicated by double square brackets defines an account.
Here is an example configuration
[accounts]
[[work]]
realname = Bruce Wayne
address = b.wayne@wayneenterprises.com
alias_regexp = b.wayne\+.+@wayneenterprises.com
gpg_key = D7D6C5AA
sendmail_command = msmtp --account=wayne -t
sent_box = maildir:///home/bruce/mail/work/Sent
draft_box = maildir:///home/bruce/mail/work/Drafts
[[secret]]
realname = Batman
address = batman@batcave.org
aliases = batman@batmobile.org,
sendmail_command = msmtp --account=batman -t
signature = ~/.batman.vcf
signature_as_attachment = True
Warning
Sending mails is only supported via a sendmail shell command for now. If you want to use a sendmail command different from sendmail -t, specify it as sendmail_command.
The following entries are interpreted at the moment: