[xurshudyan]

priority() and headers() add a non-static closure to the mailable's own callbacks, so the mailable holds a reference to itself. After sending, it isn't freed until PHP's garbage collector runs, along with everything it holds, like attachments.

class InvoiceMail extends Mailable
{
    public function headers(): Headers
    {
        return new Headers(messageId: 'invoice-1@example.com');
    }
}

Sending 500 mails with a 1 MB attachment:

Mailable	Before	After
plain	2.0 MB	2.0 MB
with headers()	504.5 MB	2.0 MB
with priority(1)	504.4 MB	2.0 MB

Neither closure uses $this, so they can just be static. Same fix as #61802 and #61438.