Could you check on the mail delivery? I’m not sure if DKIM is enabled for example on invoiceninja.com. In my experience dkim implementation improves email deliverability a lot.
I did a test checking with http://www.mail-tester.com and it seems you are not fully authenticated. Maybe thats why sometimes mail is getting in Outlook spam box.
The mail-tester test indicates the following:
You do not have a DMARC record
A DMARC policy allows a sender to indicate that their emails are protected by SPF and/or DKIM, and give instruction if neither of those authentication methods passes. Please be sure you have a DKIM and SPF set before using DMARC.
You do not have a DMARC record, please add a TXT record to your domain _dmarc.invoiceninja.com with the following value: