I have a static 'Thanks for purchasing' email that I use to manually send to customers. Sometimes the email doesn't send to the person, but there is no error reported (ie. bounced, or undeliverable etc). The recipient simply doesn't show up in the 'sent' list.
It's not due to them being unengaged - sometimes a customer will open/click the first time I send this email, but I'm unable to send a second time to them in the following weeks.
I will tag some top experts on this matter, @louischausse, @GuyTaylor, @rikkilear, what would be your suggestions to @softdoughco question about marketing emails and deliverability to her contacts?