Subscribe By Email isn't sending to all my subscribers

I really want to use this plugin vs. the JetPack subscribe feature due to the customized emails to subscribers, however it seems not everyone is getting them. So far it feels like Google Apps users who have their own domains (vs. a gmail account) are not receiving them. Is there any known issue with this?

So far I can only confirm that this theory is consistent with the two email addresses that have not received anything from our blog yet. Thank you.