Access RDB says emails were sent successfully:
When Access RDB reports that an email was sent successfully, it means that your mail provider has accepted the email into their systems, ready to relay on to the intended recipient.
This part of the email's journey is now beyond the control of Access RDB, and you will need to refer delivery problems to your mail and/or domain provider (your IT team may be able to handle such enquiries for you).
To assist with this process, you can let them know the following:
Emails from Access RDB are sent via using the settings in Options ; User preferences ; Mail Service.
DNS records should be checked, including MX, SPF and DMARC records.
Commonly used web-based sources of mail domain black-lists should be checked to ensure that you haven't been black-listed.
You may also need to ask intended recipients who consistently don't receive your email sent from RDB to:
Check that your emails aren't being blocked by their own systems or sent to their Junk Mail folders.
Ask their own respective email providers or IT teams to white-list your email domain so that they are not blocked.