Factors that can cause emails to end up in your spam/junk folder include:
- Sender name and email address matches the name on the destination mailbox - it's common to see spam to come from the person's name
- The domain was only registered recently - most popular mail providers won't trust domains until they're sufficiently aged, ie 30+ days
- SPF records not set, or set incorrectly - please ensure that you set your SPF are set correctly for your forwarded emails to work
- DKIM records set incorrectly - incorrect DKIM records will result in delivery failures
- Lower cost TLDs are often used by spammers, resulting in that extension being less trusted or blocked by providers - see Spamhaus' list of the worst TLDs here
- Microsoft and Proofpoint are very difficult with their spam filters, so any of those factors are more likely to cause issues when delivering to their customers
