PHP Notice: Undefined index: o:tag

Same problem for me, but on a single WP site on PHP 5.6

Fixed this by changing line 154 to:

if (empty($body['o:tag'])) {

Not sure if that breaks any functionality, but it seems to be working for me

Scratch that, I think that may have broke sending

Hi everyone.

I am no PHP expert, but it appears on line 154 when $body[‘o:tag’] is referenced, $body[‘o:tag’] is in fact undefined. Which is why what Phil said ‘worked’. However, preventing that entire body from executing could cause problems. A better workaround, I think, would be to provide the default option that line 154 wants, which is when $body[‘o:tag’] is equal to ” rather than being undefined.

In short, go to lines 146-149 and change

if ( isset( $mailgun['tag'] ) ){
		$tags = explode(",", str_replace(" ","", $mailgun['tag']));
		$body['o:tag'] = $tags;
	}

to

if ( isset( $mailgun['tag'] ) ){
		$tags = explode(",", str_replace(" ","", $mailgun['tag']));
		$body['o:tag'] = $tags;
	} else {
		$body['o:tag'] = '';
	}

This makes sure that $body[‘o:tag’] always has a value and will not be undefined when it is referenced. Hopefully this helps!