Can not send email

I've got the below code which will send me an email of error details when dealing with customers. The problem is the email only seems to successfully send from certain ISP's. Could it be that even though the correct authenication details have been eneterd that certain ISP's are blocking port 25?

vb Code:

Dim SMTPUserInfo As NetworkCredential = New NetworkCredential("[email protected]", "MyPassword")
            Dim insMail As New MailMessage(New MailAddress("[email protected]"), New MailAddress("[email protected]"))
 
            With insMail
                .Subject = "Error logs for customer " & customerID
                .Body = strBody.ToString
            End With
 
            Dim smtp As New System.Net.Mail.SmtpClient
            smtp.Host = "mail.OurWebsite.com" '
            smtp.UseDefaultCredentials = False
            smtp.Credentials = SMTPUserInfo
            smtp.Port = 25
            smtp.Send(insMail)

Most ISPs won't relay messages for other domains. It's a measure against spammers and those who would send messages impersonating other parties. If you send a message from your own machine using your ISP's mail server then it will send fine. If someone else tries to send a message from another ISP using your ISP's mail server, it will most likely be rejected.

You'll have to either use each individual user's own mail server to send the message or else use a mail server that will accept the message no matter what network the user is on.