Hi @Mahendra Reddy,
Thank you for posting in this community.
Since we can't reproduce your situation, we need to narrow down the problem and try some things. So, please kindly provide more information below to let us work further.
1.Verify that outgoing email settings are configured correctly in Central Administration. Go to Central Administration > System Settings > Configure outgoing e-mail settings and ensure that the correct SMTP server and port are specified.
2.Check if the email address associated with the site collection is valid and active. You can do this by going to the site collection settings > Site collection administrators and verifying that the email address is correct.
3.Please apply that custom receiver to other document libraries and see if the problem still occurs.
Thank you for your efforts and time in advanced.
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