error on delivery for such email

hi,

i have Exchange server 2019  deployed and everything is working fine  , but i have  a user when he try to send always  his mails not delivered to outside  , he can send mails from same domains  or to the existing domains on exchange   but to outside domains  no   and in queue  i notice that all his messages error are like that :

421 4.3.2 188.214.135.26 sent 10 spam today, such email sending will be limited.

any idea about this issue ? and how to resolve it