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I know this message is kind of old but it's one of the first Google hits to come up when using Google to search for a specific error message when attempting to setup email sending of ebooks in Calibre.
Trying to setup Calibre on a chromebook and was testing the sending of ebooks using a kindle email account through GMX.com and got the "invalid credentials" message.
Upon looking at my GMX inbox, I had received a message from GMX that an email program was trying to interact with their servers but that functionality was currently turned off for my account.
I had to specifically enable POP/IMAP access for my GMX account. After enabling, I was able to successfully send a test email in Calibre using my GMX.com credentials.
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