There can be the case where an Outlook message is delivered, but an email may be filtered out if it has been detected as a bulk email.
The issue here is the word BLAST.
What will happen over time when sending a large number of emails to the same domain, is that the recipient mail servers will set their filters to not deliver these bulk emails internally. An additional issue is that bulk emails can be over-formatted, unlike Outlook emails, so as a result, they trip content filters. The filtering of emails is further compounded if the emails have historically not been read, or have bounced to that domain.
Here are some way to ensure emails are being delivered to email addresses you are sending bulk emails to
- If you are sending lots of emails to one domain stagger them over time
- Reduce over-formatting of emails and use basic HTML
- Maintain list hygiene by removing invalid and uninterested e-mail addresses from your mailing lists
- Contact the IT administrator for the domain not delivering to diagnose the problem
- Use a platform that can deliver email via your internal mail server so it does appear to come from inside your domain (unlike a bulk email)