I have been having a recurring authentication problem with emails on my account when using third party email clients. It doesn't matter if the client is Spark, Outlook, emClient or any other. It also doesn't matter if I use the base or "pro" (as in imappro) servers. It also doesn't matter whether I use my main password or an app-specific one.

Everything will be fine and then, all of a sudden I will get authentication errors. Might be SMTP with an inability to send; it might be IMAP with an inability to receive. It might be both and it might be neither. And that's the point. It's intermittent. It will go on for a few days and then stop as inexplicably as it started. And then do it again. Over and over and over.

Very frustrating. 

Plus: I sent an email to Zoo support yesterday. Not even an automated response acknowledging receipt.