Mails sent to 3 fixed addresses always arrive at #1, sometimes at #2 and/or #3

Hi,

WordPress version 4.9.1 on a shared DreamHost account
Contact Form 7 version 4.9.1

I’ve created a WP site for a non-profit organization.
A few months ago it was brought to my attention that a mail sent from the contact form was not received.

I then double-checked the Contact Form 7 configuration, but couldn’t find anything wrong.
To prevent the loss of emails, and help investigate this issue, I made two changes:
– I installed the Flamingo plugin, so I can see which mails are sent.
– I added two extra email addresses as receivers, so mails are sent to three addresses now.

Contact Form 7 configuration:
To = [email protected], [email protected], [email protected]
From = [your-name] [email protected]
Additional headers =
Sender: [email protected]
Reply-To: [your-name] <[your-email]>

Checking against Flamingo I see that thus far all mails arrived in GMail ([email protected]).
Sometimes the mail arrives at one of the mydomain.nl or the mail.com accounts, sometimes both, sometimes none receives it (also not in Spam folder).

I’m puzzled and clueless…
Any hints on how I can further investigate this?

Cheers,
Rene