As of K9-Mail 6.400 on sending emails off Yahoo I get a 6.6.0 error message. What does it means?Anyone else? Any workaround?
Googling “yahoo 6.6.0 error message” brings up a ton of results, have you looked at any of them?
Otherwise more details will be needed: Device, Android/Rom version, are you using OAuth or not, LOG
Thank You for joining in. Yes I did, nothing useful nor specific to K9. Also googling for K9-Mail 6.6.0 error was of no use. Rolled back to K9 6.202 and everything is back to normal. Will test further and report back.
Welcome to the forum, @luca.mg 
K-9 Mail is displaying the error message returned by the outgoing mail server. “6.6.0” is not a standard error code. So you’ll have to ask Yahoo what this is all about.
In general, please try to add as much information as possible when asking for help. The actual error message, for example, might contain useful hints.
This support thread suggests that Yahoo doesn’t allow sending messages without a “To” recipient: "An error occurred while sending mail. The mail server responded: 6.6.0 Error sending message for delivery.. Please check the message and try again." | Thunderbird Support Forum | Mozilla Support
Have you ruled that out as the cause for your issue?
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Well as weird as it could seem I ugraded to 6.400 again, and Yahoo on top of K9 is back to normal. Either it was Yahoo momentarily down, or uninstalling 6.4 to roll back to 6.2 cleaned up some config mess. Thank You everybody for Your time.