Is it ok if I put an email address in "About" of application, and every user who asks for the source code emails me and I send him/her by email?
It’s an Android application and I will distribute it using the Play store and other similar stores.
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Technically it might be allowed, but only if you distribute your application by email. While email isn't a generally allowed form of source code distribution, using the same distribution method for both binaries and source is allowed.
The logic is simple: if you emailed the binaries, the customer obviously has email, so you may rely on further email distribution. You can't assume the user has email when he downloaded the binaries via a play store.
Note that it is valid to include the source code directly with the play store download. Also, extra download methods are always allowed: "You may convey verbatim copies of the Program's source code as you receive it, in any medium" [Emphasis mine]. Email isn't banned; it just cannot be the only method.