I usually stay away from threads like this but I just couldn't THIS time.
Both my daughters have mild dyslexia and that does NOT mean they are not intelligent children. My oldest DD has a passion for writing and although anything she writes is full of spelling and grammatical errors, her opinions are well thought out and credible.
Even so, out in the real world, she will eventually be judged on her ability to communicate with the appropriate spelling and grammar.
Spelling is not related to intelligence, I agree. That perception is out there, however, and people who not only cannot spell, but do not correct their own mistakes, are seen as unprofessional. This may not be ideal, but it is the way things are.
A company cannot expect its customers to understand that this or that employee may be dyslexic. They will see a letter full of errors and judge the company accordingly.
I routinely received job applications that are full of spelling and grammar mistakes, and I throw every one of them straight into the trash. It's carelessness, in my opinion, to not look up the correct spelling of a word.
On this board, many misspellings can be forgiven as typos, something I have been guilty of myself. When I see someone repeatedly make the same simple error however, (writing "loose" for "lose" or "advise" instead of "advice", for example) I have to conclude they are simply to lazy to care how they come across to others.