I got an error in system.log
main.ERROR: Environment emulation nesting is not allowed.  
Why this came and how to fix this?
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First, you need to find out that from which email this issue happening.
If you have using
startEnvironmentEmulation() in your custom email then you should ensure that run
stopEnvironmentEmulation () for
emulated environment after doing your work.
stopEnvironmentEmulation is not called at your code.
See some reference: