We need to narrow the question down to whether the email is failing or the donation is failing, and therefore not sending the email. The best way to tell is look at the log. If a donation completes successfully, it should show the following lines for each donation:
2016-03-31 22:56:46 PayPal reports payment status: Completed
2016-03-31 22:56:46 Creating donation from transaction audit data in 4.x mode.
2016-03-31 22:56:46 Creating donation record…
2016-03-31 22:56:47 …of type donation
2016-03-31 22:56:49 Created donation 178 for session ID SDS01-9A1CF3B4-B957-BFA7-AD78-2C2BF9554AE6
2016-03-31 22:56:49 Payment currency = USD
2016-03-31 22:56:50 IPN processing complete.
You want to look for PayPal reporting completed, the donation record added, and so forth. If all that is done, and you see the donation in your dashboard, then suspect email. If you don’t see all that, then you’re not ready to look at email.
As for email, because every WordPress hosting environment is different, the built-in WordPress email capability often is somewhat unreliable. There are plugins that use external mailing engines and substantially improve mailing reliability. Visit the Seamless Donations Plugins and Extensions page for some great, free email-helper plugins: https://zatzlabs.com/project/seamless-donations-plugins-and-extensions/.
Regarding deleting entries, Delete Monster is required. Because I’m still trying to get Seamless Donations to the point where it’s fully self-supporting (and because it’s potentially dangerous), I put that code into an add-on.
If you’re curious about the story behind the add-ons and Seamless Donations, feel free to read my one-year anniversary story at https://www.zdnet.com/article/a-year-later-what-happens-when-you-adopt-an-open-source-project/. For even more background on the add-ons and why I’m charging for them, read https://zatzlabs.com/coding-vs-yard-work-and-a-swimming-pool-would-be-nice/.
Thanks for understanding.
–David
P.S. And, join me at Jacksonville WordCamp. I’m doing a whole talk on adopting plugins.