There is a problem with the auto-correct feature as it cannot recognize homonyms such as their, there, and they're. In addition, archaic but correct spellings are often recognized as incorrect by the auto-correct features we use today.
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Even the words you, your, you're, yours, our, and ours are often not corrected by the auto-correct features because the words are spelled correctly in themselves, but they are not spelled correctly in the context in which they are used. Thus, the auto-correct feature will not flag many word as incorrect.
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In addition, as one reaches old age, with blurry vision, astigmatism, brain injury due to TIAs or strokes, and/or presbyopia, or as one develops pre-diabetes or diabetes, which is very common today, one's vision deteriorates and a person is more dependent on these auto-correct features.
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Furthermore, some people are legally blind from disease or injury, or they were born that way and must dictate their communications, which are totally dependent on an auto-spell feature, which also makes mistakes. I have communicated online with several individuals who are legally blind from birth, and it amazes me that they are able to participate in board discussions as well as they do.
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Then there are those who are dependent on their smart phones using tiny buttons and who are typing while driving. Not only are they endangering their lives and the lives of all them around them, but also they often use text message abbreviations or auto-dictate features that can drive prescriptive grammarians batty.
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Finally, those of us who are trying to revise our posts may make a change such as reversing our clauses. This leads to grammatical errors, which are not spelling errors but awkward construction errors.
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Those of us who are vision-compromised do try to correct our spellings within the 15 minutes allowed by Matthew's board, but sometimes, we receive a phone call or must answer the door bell, and then unfortunately, we fall outside the time to edit. And, yes, it is very embarrassing and humbling to see those glaring typographical errors. All of us in the end, must depend on the charity of others to be patient, loving, and kind as St. Paul recommends.
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I have found that books such as There is No Cow in Moscow can be very helpful. There are online help guides, too.
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Nadir, what online websites do you recommend?

This is not a spelling question but a pronunciation one.

When saying the word blessed,  when do we pronounce it as spelled and when do we pronounce it
as bles-said, as in the Hail Mary.
Blessed would fit perfectly in the Hail Mary (as one who has been blessed) yet we pronounce it
bles-said.