Canon MB5400 Series Online Manual page 1085

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Cause
Communication with mail server failed.
What to Do
Tap OK on the touch screen of the printer.
Make sure that the settings for secure connection (SSL) of your mail server and printer are match.
For printer's settings for secure connection (SSL), use Remote UI.
Setting Mail Server
For setting for secure connection (SSL) of your mail server, contact the administrator of mail server.
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