Requested action aborted: error in processing
When attempting to process the message, the receiving mailserver's rules prevented it from processing it for delivery. Rules such as SPF verification can create this error.
Depending on the exact message given, it's most likely related to the recipient server's rules. For example, SPF verification requires a DNS lookup for the domain the email is being sent from. If the DNS server for that domain does not work or fails, the mail server can not process the rules.
Check and make sure that whatever email address you are sending from (eg. email@example.com), the domain's name server is working properly.
SMTP error 451 Unable to complete command, DNS not available or timed out
451 Domain of sender address does not resolve