"Broadly speaking, any merger creates chaos, and the spam situation might well get worse in the preliminary period," said Scott Hazen Mueller, chairman of the Coalition Against Unsolicited Commercial Email (Cause).
Steve Linford, chairman of anti-spam organisation Spamhaus, said: "Verizon has very quickly moved up to eighth on our list of ISPs that host spam organizations.
"Its deal with MCI will create the biggest spam-hosting ISP on the internet. It's disappointing."
