Emails disappearing into the void of the Internet is a common issue. There are many logical and mysterious reasons why your message doesn’t arrive where you intended. Typically there’s not much you can do about email deliverability. All the email services have tightened up their security to scrutinize whether inbound emails are legitimate.

So what can you do?

You should try to determine a pattern — for example, your emails sent to @aol.com or @yahoo.com are always disappearing.

Are you’re using Google Apps email with your domain and experiencing this problem?

Assuming that the domain name and specific email account isn’t being used to send out high frequency spammy emails, the only thing I can suggest is to assign a Sender Policy Framework (SPF) that applies to all emails sent from your domain.

This explains what it is/does:
https://support.google.com/a/answer/33786?hl=en
This explains how to set up the TXT record with your specific domain name registrar:
https://support.google.com/a/topic/1409901

Create a TXT record containing this text:
v=spf1 include:_spf.google.com ~all

Add SPF Record to your domain name zone file

Allow 24-48 hours for that to propagate / work. But there’s no guarantee, it’s just plugging a possible leak.

An extra precaution… Add a digital signature to your email header.

Email spammers may pretend to send email from your address. If you setup DomainKeys Identified Mail (DKIM), it adds a digital “signature” to the header of email messages sent from your domain, so recipients can verify that the message really comes from your domain. Here’s how:
https://support.google.com/a/answer/174124?hl=en

Add DKIM Record to your domain name zone file

Still having delivery problems?

  • Definitely try to determine a pattern (e.g., emails sent to @aol.com or @yahoo.com are always disappearing).
  • Do a specific test with that person again in real time to see if they receive it.
  • Make sure there isn’t a typo in the email addresses.
  • If you’re paying for Google Apps for Work or using the Education/Nonprofit Editions, log into your Admin Console as an administrator and look for the Google Support contact phone or live chat option.
  • Ask the person who does your I.T./email support who might know another way to trouble shoot this.

I hope the above was helpful for you.
-Roland