[SalesForce] Send Email from Apex Class in Sandbox not working

I'm wondering if anyone can help. I have a trigger that uses a helper class to send an email using the sendMail method. This code is written in one of my Sandboxes and when I test this functionality through my test classes and check the debug logs it looks like the send is successful.

I have also tested this by turning on the debug logs when doing a manual test through the Salesforce UI, again the debug logs show a successful send to my email address, however, I don't receive that email in my inbox.

I have tried to resolve this myself by looking up solutions from other people who have reported similar problems but so far I have been unsuccessful.

Deliverability is set to 'All Emails' and I have changed my email address in Sandbox to ensure that emails get sent to my actual email address. I have been able to receive other notification emails from my Sandbox but any time I try to run my apex code by inserting a duplicate lead with the same email address (this is one scenario), the addError appears on the record page and the logs indicate that the email was sent, but I don't receive the email and requesting email logs for the time it was sent don't show the email as being sent out of Salesforce.

Here's my code snippets:

if(existingLeadsByEmail.containsKey(l.Email){
    matchOnEmail = true;
    buildEmailMessage(existingLeadsByEmail.get(l.Email), l);    
    l.addError('A lead with this email address already exists.'); 
}

The buildEmailMessage creates the email and adds it to a list of emails to be sent after all incoming leads have been processed. Then to send the email I use the following:

if(allMails.size() > 0){
    List<Messaging.SendEmailResult> results = Messaging.sendEmail(allMails); 
    if(!results.get(0).isSuccess()){        
         System.debug('Send Error: ' + results.get(0).getErrors()[0].getMessage());     
    } else {        
         System.debug('Email Send Successful!');    
    } 
}

Any help on this would be greatly appreciated.

Many thanks guys!