wp_mail() method already exists from a different plugin

Question

I am using the Mailster plugin to send newsletters and also for system emails on my WooCommerce website.

After a recent update, I am getting this error message

The wp_mail() method already exists from a different plugin! Please disable it before using Mailster for system mails!
More info: - /wp-includes/pluggable.php:174

But I don't know of any plugin that uses for email purposes.

Most importantly, isn't pluggable.php is one of the core files of WordPress?

Any pointers to resolve this issue is much appriciated.

Thanks!