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 = "abc@comcast.net", txtFileAttach AsString = ""

txtFileAttach = "Weekly_BU_Report_" & Yr & "_" & Mo & "_Gen_" & FormatDateTime(Now(), DateFormat.ShortDate) & ".xls"
'*** pull 'to' recipients...

SMTP Email not reaching all recipients