This article discusses the fact that the size of a digitally signed and encrypted e-mail message grows when you create, forward, or reply to a digitally signed and encrypted e-mail message in Microsoft Outlook. This growth continues even if you do not use digital signatures or encryption in later replies and forwards.
When you digitally sign or encrypt a message, the message is stored as MIME, instead of as MAPI. This process causes all attachments to be stored in base-64 encoding. This process also causes an increase in attachment size by a factor of 4/3.
When you digitally sign and encrypt a message, Outlook first signs the message. This causes a base-64 increase in the message size by 4/3. Outlook then encrypts the message. This again causes a base-64 increase in the message size by 4/3. Therefore, the message size grows by approximately 16/9. Additionally, there is some overhead that is associated with these two processes.
If you use Rich Text Format (RTF), the message is stored together with two copies of the message's body, the HTML formatting, and the RTF formatting. This also causes the message's size to grow.
Additionally, the digital signature or encryption data is added to the message, together with all the included overhead, when you send the message. For digital signatures, this process includes the certificate chain, the signing time, the S/MIME information, and so on. For encryption, this process includes the lockboxes for each person for whom the mail is being encrypted. This also causes the message's size to grow.
Plain text is much smaller than HTML. When you use plain text, the growth rate does not exceed 50%.
When you exceed Exchange Server Message Size limitation set in the Outlook Client you receive the below error message:
"There is not enough memory or system resources. Close some windows, and then try again."