Since I write to my friend almost every day I thought that I could write "I use to write" while "I used" means that I don't write mails anymore ....not quite sure if it is correct though
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I used to write refers to what you wrote in the past. I use to write is not the present tense of I used to write.
We can write 'emails', but not 'mails' because 'mail' is an uncountable noun.