If you are an AOL customer, and you are shown the '521 5.2.1 : AOL will not accept delivery of this message' error, then please make sure that your sender Identity name in Mailbird does not include the word 'AOL' (regardless of whether it is capitalised or not), since AOL does not allow this.

Another common error is '521 5.2.1 MUST COMPLETE IMAGE PUZZLE BEFORE SENDING. PLEASE GO TO http://challenge.aol.com/spam.html' - this error will be shown the first time you try to send mail through AOL from a email client such as Mailbird. To solve this, simply click on the link to complete the image puzzle provided by AOL, and you should then be able to send emails once again afterwards.

Hopefully you have found this article useful, but if you still require assistance, please do not hesitate to contact our Customer Happiness Team by clicking here.