From the z/OS Communications Server Bookshelf, manual IP Configuration Reference
section 30.29 on SMTP configuration parameters:
Use the MAXMAILBYTES statement to specify the maximum size in bytes of mail that is accepted over a TCP connection. Reply code of '552 Mail file too large' is sent to the remote SMTP client if the number of mail bytes arriving exceeds this value. This value is also used to determine the space allocation requirements for the data sets which hold the mail during processing (see Usage Notes in this section below). These data sets names are &mailfiledsprefix.*..NOTE and occupy a minimum of 2 tracks per data set.
The default value is 524288 (512K) and applies to both sending SMTP messages and receiving SMTP messages.
If you are getting a spool truncated message, however, the truncation could be occurring via JES. This is an issue where you can communicate with us just as much as you care too but you will not get any resolution to your problem here. The ONLY way to resolve your problem is to talk to your site support people
and find out (a) where the truncation is occurring, (b) if they can change parameters to allow your message to be emailed, or (c) if you have to find another way (such as FTP) to accomplish the task.