SMTP Email not reaching all recipients

This code started having problems last week - after about 2 years of no problem, 2 recipients out of 5 aren't receiving the email. They are on 2 different domains, so it's mysterious. Anyone have any suggestions on what to try? Thanks in advance for any help!!

Code:

Try
Dim strFrom = "[email protected]", txtFileAttach AsString = ""

txtFileAttach = "Weekly_BU_Report_" & Yr & "_" & Mo & "_Gen_" & FormatDateTime(Now(), DateFormat.ShortDate) & ".xls"
'*** pull 'to' recipients from database
Dim strTo = Util.GetReportEmails("Weekly_BU_Report", "RecipientsTO")

Dim MailMsg AsNewMailMessage(NewMailAddress(strFrom.Trim()), NewMailAddress(strTo))

MailMsg.BodyEncoding = Encoding.Default

'*** Add "TO" recipients from tbReportUser table
MailMsg.To.Add(Util.GetReportEmails("Weekly_BU_Report", "RecipientsAddTO"))

'*** Add "BCC" recipients from tbReportUser table
MailMsg.Bcc.Add(Util.GetReportEmails("Weekly_BU_Report", "RecipientsBCC"))

MailMsg.Subject = "Weekly Business Unit Report"

MailMsg.Body = "Please see attached file. Please contact me at [email protected] to discontinue receipt of this report. "

Dim stream AsFileStream = File.OpenRead(FileName)

Dim attachment AsAttachment = NewAttachment(stream, txtFileAttach)


MailMsg.Attachments.Add(attachment)

MailMsg.Priority = MailPriority.High

MailMsg.IsBodyHtml = True

'Smtpclient to send the mail message

Dim SmtpMail AsNewSmtpClient

SmtpMail.Host = "localhost"

SmtpMail.Send(MailMsg)

Catch ex AsException

'*** I call utility that emails error information to me. I am not getting any errors.

EndTry