System.Exception: Mailer.SendSMTP() Failed

Question

I got below exception and i didnot understand what could be the reason. it supposed send email with an attachment of 45 mb size file. other is able to send email via same smtp server. please help me. thank you.

System.Exception: Mailer.SendSMTP() Failed: System.Net.Mail.SmtpException: Mailbox unavailable. The server response was: 5.7.0 Message Size Violation at System.Net.Mail.DataStopCommand.CheckResponse(SmtpStatusCode statusCode, String serverResponse) at System.Net.Mail.SmtpConnection.OnClose(Object sender, EventArgs args) at System.Net.ClosableStream.Close() at System.Net.Mail.MailWriter.Close() at System.Net.Mail.SmtpClient.Send(MailMessage message)