From a reformed pedant, things I've learned as a 60 something that have made me more accepting of mistakes, including my own:-
Some people dictate their emails as they are unable to use their hands, spelling mistakes are not their mistakes (try it some time).
People have dyslexia.
Typos happen.
Too few staff working in a busy office and none of these people actually learned to touch type so will make mistakes but time constraints stop them checking things properly. Plus the staff are tired as are always overworked. Autocorrect has changed some words to Americanised spelling.
Some people become unwell with illnesses that affect their cognitive abilities, spelling suddenly becomes confusing, the person is ill and tired but is under pressure to get something finished and doesn't see own mistakes. (Might be you one day, it's happened to me).
Reading too many American novels on a Kindle causing self doubt when spelling certain words.
The list is endless. If you understand what someone is actually trying to convey, does it really matter? However, I would expect letters from schools, universities, solicitors' offices and professional bodies to not contain mistakes.
One last thing worth remembering - some misspellings are indicative of scams, be discerning in judging this but if in doubt then regard it as such until you know otherwise.