Error message screen shot:
For this issue you need to check the SAP note 690020.
As per SAP note 690020:
If you want to allow larger emails to be sent, increase the mpi/total_size_MB parameter in the SAP profile to a value that is
greater than a quadruple of the largest mail to be sent. (For the size of the mail, as a rule of thumb you can use approximately 150% of the
size of the user data contained in the mail).
And you can see this in transaction SOST:
No delivery to because message too long, maximum length 20,000,000 bytes
Message no. XS841
The message could not be delivered to recipient because the message text was too long. The node used supports messages up to a a length of
20,000,000 bytes. The message text is, however, 95.181.977 bytes long.
You need to check on your system the value of the parameter mpi/total_size_MB, if its value not large enough (default value is 80), you need to increase
it to 96 x 4 = 384 (in this case it's 95.181.977 bytes long) .