Policy
Set Maximum Certificate Size Limit for Signed Emails in Reply Scenarios.
This policy setting controls the maximum limit for the size of the stored certificate in Exchange server when replying to a digitally signed email using Outlook. If you enable this policy setting, you can configure the maximum limit for the size of the stored certificate, up to a maximum of 16384 kb. This helps to avoid issues where a reply to a digitally signed email may not be delivered or saved in the sent items due to large certificate sizes of the original sender. By disabling this policy setting, the maximum limit will be the default set at 12921 kb, limiting the ability of administrators to adjust the maximum size of the stored certificate. If you do not set the policy setting, it is treated as enabled and the default maximum limit for the size of the stored certificate is set at 12921 kb.
cd035c42f875 L_SpecifyMaxCertLengthAllowedtoStamp Registry
Copy registry mappings
HKCU\software\policies\microsoft\office\16.0\outlook\options\mail\maxcertlengthtostampPolicy notes
This policy setting controls the maximum limit for the size of the stored certificate in Exchange server when replying to a digitally signed email using Outlook. If you enable this policy setting, you can configure the maximum limit for the size of the stored certificate, up to a maximum of 16384 kb. This helps to avoid issues where a reply to a digitally signed email may not be delivered or saved in the sent items due to large certificate sizes of the original sender. By disabling this policy setting, the maximum limit will be the default set at 12921 kb, limiting the ability of administrators to adjust the maximum size of the stored certificate. If you do not set the policy setting, it is treated as enabled and the default maximum limit for the size of the stored certificate is set at 12921 kb.