Issue a command called CHGPOPA and change the Message split size to *NOMAX. This will resolve your problem.
Don’t use SNDDST if you’re not sending SNA distributions, which is what the command does. Just because it can route some messages across the gateway to the SMTP server doesn’t mean it can route everything across.
If you want to use the SMTP server, then use the facilities that are intended for it. Use SNDDST only for what it’s supposed to be used for.