念のためにメルアド教えとくねって英語でなんて言うの？

Just in case- I'll give you my email address just in case I'm not on Line
Highlights that they might need it and is a polite way to give it to someone.

You can add " I'm not on Line" to the end of the sentences to justify why you're giving the person your email address.

"Here's my email address in case you can't get me on LINE!"
"If I am not available on LINE, here's my email address!"
"in case" meaning that if something happens you can be reached with a different method of communication.

The above examples mean that if you can't be reached via LINE then the person can contact you with your email address.

To someone you are texting on LINE and you want to tell them your email address, just in case, for some reason they can't contact with you by phone, then you may just use one of the above exampled sentences.

When we give someone our contact information, we often just use the word "here" to indicate that we are giving it to them. We use the verb "reach" to mean "contact." I hope that this helps. :)

The address you send emails to is call your 'email adress'
when you are telling someone something that they don't know but might need to use in the future you would say 'just in case'

You can use the expressions "contact me" or "get a hold of me" to talk about someone trying to speak/text/call/email you about something.