Post a notice on your web site and possibly as a stuffer in advance so they know it is coming and that this is an extra step in the communication process, but it is intended to make them more secure. You should also do this in an email blast. Phishing scams have a short life-cycle. By sending a notice that says "in the next 30 days we'll be... and you should register, call if you have questions..." you help eliminate the doubts because phishers wouldn't go that long.
_________________________
AndyZ CRCM
My opinions are not necessarily my employers.
R+R-R=R+R
Rules and Regs minus Relationships equals Resentment and Rebellion. John Maxwell