Error: 550

Error: 550

If any client gets the error like '550 relaying to prohibited by administrator', it generally occures because of the IP Blocking, so you can give them the following reply:

Some internet services block third party email servers from functioning. Their reason is that they want to prevent you from using these servers to send SPAM (junk mail). In this cases you have two options. You can either contact the internet service provider (ISP) and have them unblock your account, or you can use your ISP's SMTP server instead of your domains'. When you send email, it still appears as if it is coming from your domain and it makes no difference.