The most common type starts with: Hello, you're missing 300 visitors a day.
The next most common type: Hello, I hope your day is fine. There are some changes we could make to your website to convert more visitors.
They all come from some non-trustworthy email address, something like this:
FirstNameSEO@yahoo or google, no phone number, no website, no anything to check them out.
The first type lose me as soon as they say 300 visitors a day. But even if I could pass that, I can't see myself ever working with someone who's sending me an email out of the blue with nothing to check them out, I just trash.
Occasionally, I get email messages about changing something on one of my sites from someone with
company phone number
Company fax number
And, the email address is something like JimD@companyName.com.
I instantly assign them way more credibility than I do the others.
That's before I read their message!