Liath on "[Plugin: WP Mail SMTP] How to remove the X-Priority header?"

you have to set the priority to NULL

i don't know this plugin, found your question on google :)

when there are no option to set the priority to NULL, you have look into the code for something like this

$result .= $this->HeaderLine('X-Priority', $varforpriority);

and set $varforpriority to NULL

this will remove the header