ISP AAPT has sent blank emails to customers in an attempt to advise them of the impending sale of their consumer broadband or phone account to iiNet.
Multiple copies of the email were sent to customers on Wednesday but many who opened it saw only an "important message" from AAPT customer service - with nothing inside.
A representative for AAPT confirmed the issue for users of Hotmail services - but iTnews also sighted similar problems for users of Yahoo! Australia's free email services.
"Apparently some customers who use a hotmail email address are not able to view the email we sent," the representative said.
"This appears to be an issue with hotmail, but the content of the email should be able to be viewed in the helpyourself section of www.aapt.com.au under 'your messages'."
The email said that although it had sold the consumer division to iiNet, it would be business-as-usual for customers with respect to services and billing arrangements.
iiNet paid $60 million for AAPT's consumer business. The acquisition was expected to be completed by the end of the month.