Question:
Large E-mails from InterForm400 are split up into several E-mails. How can I prevent that?

Answer:
It has most likely nothing to do with InterForm400.

The problem is probably, that the E-mail size is 128 Kbyte or more. By default the iSeries split-up mails into pieces of 128 Kbyte. The 128 Kbyte can by increased up to 2048Kb or even *NOMAX with the command, Change POP Server Attibutes:

CHGPOPA MSGSPLIT(*NOMAX)

(Here the value is increased to *NOMAX).