The acquisition, conditioned upon customary regulatory approval, is expected to close by early July.
CEO Enrique Salem, who previously worked at Symantec, said the move would strengthen the company but hinted there would be an impact on BrightMail's current operations.
"In these situation there are always redundancies," he said. "Symantec is making a huge investment in BrightMail. My plan is to become part of the new company. I expect to continue running this group and expand my responsibilties."
Symantec made a strategic investment in Brightmail in July 2000 and currently holds an 11 percent equity stake in the company.