The notifications module will, if so configured, send email on node_save() - this can be perilous when testing migration into a site with notifications enabled. I lobbied Moshe to include a debug mail system class in devel:, and made use of it on my current project, but never did get around to documenting how to do this publicly. And that's for the best - as the project approaches deployment, we'll need to disable devel, but we don't want the final migration to throw emails. Also, that mail class logs all emails to /tmp, and we ran out of disk space after doing multiple full test migrations. So, I think Moshe's initial instinct was correct - Migrate should implement its own mail system class, enabled by default; and, in addition, it should /dev/null the emails.