My question

When I communicate with foreign people through for using e-mail, I often have one question, that "why do they use lower case I" small letter at the beginning of an email? 
For example " i am "
When I was in junior high school as a student, I learned about English gramma, one rule is rule as follows: 
If we use English after the period and at the beginning , we have to use capital letter .
Example "I am "
But some foreign people use lower case small letter instead of capital letter .
Why?
Perhaps, you dont need to it is needless to obey the rule strictly ?
Perhaps the case of this phenomenon is that a lot of people dont care forefeet ?
I haven't the slightest idea about it .
But I feel that it is better to make proper use of the small and capital letter.
Because I think it is easier to read .
I want to ask anyone .

The correct way to use 'I' is by adding in a capital to it. On test papers, you can get marked down on using a lower case 'i' so it is important to do an upper case 'I'

Online though, unless they are talking buisness with someone important, many people will use a lower case 'i' as it becomes easier to do. In a formal letter or email, an upper case 'I' is highly recommended.