There could a problem with the SMTP server you using to send out emails. he server could be down or blaclisted. If it is on shared hosting the server's IP could be blacklisted. If it is you would need to wait for blacklist to expire. The error you have sent does not give much details to give the correct information on resolving the issue. To give a detailed answer to what the problem is kindly send the error that you receive when you try to send out emails.