"It is high time to visit xyz headquarters if the mail won't come within 2 days."
Please correct this sentence. Please explain the usage of won't and doesn't.
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Normally future simple tense will not be used, unless the modal verb "will" means somebody's willingness to do something. See this example,
"If the post office will deliver you the mail within two days, it is...."
It means "if the post office decides they will deliver you the mail"
You can search the book "oxford english grammar" in google where this usage is explained.