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When you see this error when attempting to send a message, are you using a Fastmail domain for your From address, or another domain?
For example, if you are forwarding messages from a Gmail account to your Fastmail account and then try to reply from the Gmail From address, Fastmail has to log into the Gmail SMTP outgoing server using your Gmail credentials. If anything has changed on the Gmail end (such as your password), this will fail until you set the Fastmail sending identity up again.
If use of an external domain isn't the issue, please try this: Log into your Fastmail account and click Compose. Send a message to your Fastmail account so that the From and Send addresses are identical. Does this message get sent and does it arrive properly at your account?
Bill
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