Emails not being sent

Hello, i’m having issues sending emails from the system and it’s giving me the following errors. How can this be fixed please:

System.Net.Mail.SmtpException: System.Net.Mail.SmtpException: Syntax error, command unrecognized. The server response was: 5.7.0 Mailgun is not loving your login or password
at System.Net.Mail.RecipientCommand.CheckResponse(SmtpStatusCode statusCode, String response)
at System.Net.Mail.SmtpTransport.SendMail(MailAddress sender, MailAddressCollection recipients, String deliveryNotify, Boolean allowUnicode, SmtpFailedRecipientException& exception)
at System.Net.Mail.SmtpClient.Send(MailMessage message)
at ManagerServer.HttpHandlers.Businesses.Business.Email.Post()

Please follow the troubleshooting guide here Troubleshoot email issues including the guides to the specific email service you are using, if it is listed

Also you need to explain was it working before?
If so, what did you change?
What edition and version of manager are you using?
What platform and Operating System are you using?