We came across your contribution entitled <name of your paper here> published in <journal name here> and thought your expertise would be an excellent fit for <name of this unrelated – to your specialty – congress>.
We invite you as speaker at <full name of congress with dates>.
It is becoming a new form of either spamming or phishing (I still haven’t decided yet) or actually maybe a combination of both, especially that most seem to come from meeting organizers on topics that are completely unrelated to my field of expertise for the said meeting. I now received 7-10 of such invitations per week.
The funny thing is that even when the invitation is actually related to your field of expertise, the use of “invited” speaker (or sometimes “invited as honorable speaker”!) does not actually mean what you think it is… I did the test a few times, replying that I was “honnored” to accept the invitation and that I was expecting as an invited speaker that my registration fee and travel cost would be taken care of. It lead to interesting exchanges where they had to admit that the language used was a bit…let’s say overstated! What you are invited to is summit an abstract for the conference but you are not an invited speaker.
Even worse are the too frequent invitations (1-2 per day now) to publish in new “open” journals that are not in your own field of research: just in the last few days, I have received invitations to publish in (these were the funniest I received relative to my field of study): a nutritional biology journal, a hydrology journal and a Business and Management journal!
I mean come on, if you want us to take you seriously, better do your homework here and target those that could actually be interested in your journals! These untargeted e-mails just show us how unprofessional you actually are. In fact, your “new” journals should probably find itself directly on Beall’s list of predatory scholarly open-access publishers. I am more than happy that my SaneBox service put your e-mails away from my inbox to start with.
Still, the level of the background noise seems to be higher then ever. Just because the tools at our disposal make it easy to “communicate” for tens, hundreds or thousands at once, doesn’t mean that a minimal due diligence effort is not necessary…
This is similar to these students from completely unrelated fields (not even close) that send their CV for PhD or postdoctoral positions: seriously have you even taken only a single minute to Google me and read what I am doing?
Ruminating... 
I had exactly the same experience as you. This is getting very annoying.
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