Do you have Mcafee suite? Keep getting error 421 while transacting emails? If it is yes, then you need to fix it as soon as possible. Else, such error may put your system in serious dilemma. So, you need to Fix Mcafee Error 421 with the help of experts.
When you sync S/MIME encryption settings with MEG for both ends, then such error occurs. While sending a test email for S/MIME encryption, MEG appliance on the sender side does not work. Then, you will get message status: