What could be the reason for the "uninitialized constant ActionMailer" error in Rails?

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The "uninitialized constant ActionMailer" error typically occurs when Rails cannot find the ActionMailer module, which is responsible for sending emails in a Rails application. This can happen if the ActionMailer gem is not installed or if it is not being loaded correctly in the application. Other possible reasons for this error could include misspelling or incorrect case of the module name in the code, or problems with the environment configuration such as incorrect path or version mismatch.