Additionally, for safety purposes, we recommend using a secondary email address.
Literal
Furthermore, danger-prevention [possessive-の] for [purpose-為], secondary-address use [object-を] recommend.
なお 'furthermore, additionally' is a formal connective. サブアド is internet slang — a clipped compound of サブ 'sub' + アド(レス) 'address,' meaning a secondary email address used to protect one's main address. お奨めします is the polite form of 勧める with the honorific prefix お.