If sending mail from AOL and receiving the error message:

521 5.2.1 : AOL will not accept delivery of this message

Make sure your sender Identity name in Mailbird does not include the word 'AOL' regardless of capitalization, as AOL does not allow that.

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Another common error is:

521 5.2.1 MUST COMPLETE IMAGE PUZZLE BEFORE SENDING. PLEASE GOTO http://challenge.aol.com/spam.html.

Which will be shown the first time you try to send mail through AOL from a mail client. For this just follow the link to complete the image puzzle and sending should work afterwards.