Increase in the limit of recipient from 2MB to a higher value . Providing a notes.ini parameter to set the limitation to x MB can also be considered.

When a large group (Greater than 10K after resolution) of Recipients are listed in an email, the workstation mailer attempts to split the message into multiple messages using the following algorithm:                   
                                                                        
The groups are expanded & addresses are split into ~10K lists;  And a maximum of 200 such lists are allowed. For each 10K recipients item, a new message is created & pushed into mail.box. This is how the Mail Splitting works. Note 200 is the max-limit for such split messages.  If the expanded recipients cannot be contained within 200 lists (200 * ~10K
= 2Meg), then the below error is received "Too many recipients. Recipient addresses must total less than 2MB."

Please refer on the SPR # SVAA8ESGHX