It seems to use some helper classes from Beast, but not the protocol itself.
And that helper classes for example boost::beast::buffers_cat adds a little above asio::streambuf.

And IMAP complexity by itself is a "Beast" compared with SMTP.

Not from boost libraries, but have you look at these libraries if you need IMAP support.
https://github.com/dinhviethoa/libetpan
https://github.com/MailCore/mailcore2

As far as I know, most of the "modern" email apps which haven't used proprietary code are based on these libraries.

Regards,
Zhivko