Which wasn't tested. I suggest you open a bug to jmeter with a sample.
If you can't post bug I can post it for you if you give sample jmx that doesn't work.
If you can downgrade to Jmeter 3.1 it can work.
I understand this is a fairly big change with no tests but I thought
I'd submit anyway in the hope that someone had some good SMTP tests or
someone was willing to review or even just apply a subset of the
changes.
There's also existing bug reported on SMTP Smapler in Jmeter 3.2.
I got it to work in Jmeter 3.2.
I got the report stored in a file and gave the path to that file "Attach file(s)" section and its sending email successfully.
There was a change in SMTP Sampler in jmeter 3.2
Which wasn't tested. I suggest you open a bug to jmeter with a sample.
If you can't post bug I can post it for you if you give sample jmx that doesn't work.
If you can downgrade to Jmeter 3.1 it can work.
There's also existing bug reported on SMTP Smapler in Jmeter 3.2.
EDIT
You can download a fix to the issue in jmeter-3.3