Just released a new version of the WP Mail SMTP plugin. There were a stream of similar questions about one particular error message over the last few months. Hopefully this new version will provide more detailed debugging output. I’d recommend upgrading to anyone having problems. There are no bugfixes or security fixes in this update, so it’s not a priority release.
Update: I’ve released version 0.7 today as well. This version fixes a long-standing bug where the plugin would overwrite the from name and email, even if they had been set by another plugin (or WordPress itself). I recommend everyone update to 0.7 as soon as possible.