What does "dictated but not read" mean?

That means

"I spoke this message to my secretary or into my computer, and my secretary
or my computer did the writing.  I didn't read what they wrote before it got sent
to you.  So if you find any mis-spellings, or funny ridiculous words that obviously
don't belong here, it's all their fault, not mine.  I was not only too lazy or busy to
make the effort to write to you myself, I was also too lazy or busy or didn't care
enough to check it over before it went out to you."

There's a bit of my own opinion in there, but I'm sure you get the idea.