Issue while sending email with Appwrite messages

I have a php application for e-learning and I used at the beginning PHPMailer to send email to subscribers each time a new article was created. But today with the number of subscribers, articles notification are not sent to all the subscribers as there is a timeout on my php server hosted on Ionos. Currently I was testing the messaging feature of appwrite to check if I can use it instead and being able to do it asynchronously with my large dataset. Unfortunately I have just created my provider and tried a test email message but I get the error "failed" with the following message. Can anybody help me figuring it out ?

Failed sending to target kenguehunterdonald@gmail.com with error: The following From address failed: boyom5@gmail.com : MAIL FROM command failed,Requested action not taken: mailbox unavailable Sender address is not allowed. 1N8oCQ-1txmED0tjq-00z9ri ,550,SMTP server error: MAIL FROM command failed Detail: Requested action not taken: mailbox unavailable Sender address is not allowed. 1N8oCQ-1txmED0tjq-00z9ri SMTP code: 550SMTP server error: MAIL FROM command failed Detail: Requested action not taken: mailbox unavailable Sender address is not allowed. 1N8oCQ-1txmED0tjq-00z9ri SMTP code: 550