Geary is still in early development. Geary supports IMAP and can be used with Gmail. Geary is missing attachment support, searching and numerous other features.
Geary may be used with Yahoo! Mail, however it is highly unstable and not recommended for primary email accounts on Yahoo! Mail.
To learn more about the features we're working on and the future of Geary, please visit Geary's Redmine page.
Geary does not support multiple email accounts. If you would like to change the email account used with Geary, you'll need to delete your Geary database. From a Nautilus window, locate your Geary database in .local/share/geary. Delete the folder named after your email account. Upon relaunching Geary, you will be prompted to enter your new email credentials.