If you are using SPF (Sender Policy Framework), it is possible that when running in Flexi Cloud, emails are occasionally not delivered to clients. SPF is a tool that uses DNS configuration to prevent emails from being sent under your domain from unauthorized servers. The emails themselves (i.e., the SMTP protocol) do not handle this.
One of the reasons for non-delivery may be your SPF configuration. You can check the current value of your SPF record using the SPF Query Tool.
Add the following to your SPF settings: include:_spf.flexibee.eu
The resulting DNS configuration might look like this:
@ TXT v=spf1 a mx include:_spf.google.com include:_spf.flexibee.eu ~all