Error in sending mails

I′m having trouble with some sites and with others no, I did the same configuration in all. The email that I want to be the sender address is contacto@mydomain , I dont know why its trying with wordpress@mydomain, its not in any configuration.

Thanks in advance!

Connection: opening to ssl://smtp.hostinger.com:465, timeout=300, options=array()Connection: openedSERVER -> CLIENT: 220 ESMTP smtp.hostinger.com
CLIENT -> SERVER: EHLO mydomain.uy
SERVER -> CLIENT: 250-smtp.hostinger.com
250-PIPELINING
250-SIZE 48811212
250-ETRN
250-AUTH PLAIN LOGIN
250-ENHANCEDSTATUSCODES
250-8BITMIME
250-DSN
250 CHUNKING
CLIENT -> SERVER: AUTH LOGIN
SERVER -> CLIENT: 334 VXNlcm5hbWU6
CLIENT -> SERVER: [credentials hidden]SERVER -> CLIENT: 334 UGFzc3dvcmQ6
CLIENT -> SERVER: [credentials hidden]SERVER -> CLIENT: 235 2.7.0 Authentication successful
CLIENT -> SERVER: MAIL FROM:[email protected]
SERVER -> CLIENT: 250 2.1.0 Ok
CLIENT -> SERVER: RCPT TO:[email protected]
SERVER -> CLIENT: 553 5.7.1 wordpress@mydomain.uy: Sender address rejected: not owned by user [email protected]
SMTP ERROR: RCPT TO command failed: 553 5.7.1 [email protected]: Sender address rejected: not owned by user [email protected]
CLIENT -> SERVER: QUIT
SERVER -> CLIENT: 221 2.0.0 Bye
Connection: closedSMTP Error: The following recipients failed: [email protected]: [email protected]: Sender address rejected: not owned by user [email protected]