If you use SPF (Sender Policy Framework), you may occasionally experience undelivered emails to clients when running Flexi in the Cloud. SPF is a tool that uses DNS configuration to prevent emails from being sent under your domain from unauthorized servers. The email protocol itself (SMTP) does not address this.
One of the reasons for undelivered emails may be your SPF configuration. You can check your current SPF record using the SPF Query Tool.
Add the following to your SPF record: include:_spf.flexibee.eu
The resulting DNS record might look like this:
@ TXT v=spf1 a mx include:_spf.google.com include:_spf.flexibee.eu ~all