Thanks for creating this great a plug-in!
I noticed that the plug-in stores the SMTP password in plaintext (and the related forum messages that point out that this is necessary, since it needs to be sent to the e-mail host to authenticate the account).
Would it be worth using a symmetric encryption for the password. The key obviously wouldn't be able be formed from anything stored in the database (defeats the purpose) and would be site-specific rather than user specific.
For example it could be formed from a fixed plug-in string (appended to the database password) then hashed with
wp_hash (which uses site-specific salts). The database password & salts are stored in
wp-config.php - so the encryption would only be as secure as that file - but would protect the password if the database was stolen (or simply just viewed).
But presumably if the database was read/stolen then the password is protected?