Many support tickets to the Post SMTP plugin opened because of the error in the title.
WordPress allows me as a developer to overwrite the internal mailing flow managed by a function called: wp_mail.
this function and a lot more like her (that can be overwritten) are inside a file called: pluggable.php.
The file is included by WordPress in a specific event, and then you call each function as you want.
Many less knowledgable plugin developers try to call to a function in the file on a wrong time and getting an error message.
what do they do? instead of investigating and fix them self they doing a “hack” and including the file in each time they getting the error – BAD!!!
Because of the above when I try to overwrite the function, the file already included from one to many times causing a big conflict.
How to solve?
Disable one plugin each time until the message disappears, now you know the problematic plugin, and can decide what to do next: contact the plugin developer, find another one, etc …