Error -17099 with error message "Relaying to <user@domain.com> prohibited by administrator..." is shown when the message cannot be sent by the SMTP server.
The most common reason for this error is your ISP. Most probably their SMTP server cannot send your email because you are trying to send the message to too many recipients.
The best solution would be to reduce the number of recipients. You should avoid sending a mail to more than 15 recipients.
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