No … well, yes … let me explain: A SSL certificate is required to secure the admin area and the webmail pages. The SSL certificate is also used by (my knowledge is weak here) either Postfix or Dovecot for secure connections to the mail server.
That said, yes a SSL certificate must be acquired from a third party - a certificate authority. There are MANY different certificate authorities out there. It so happens that Gandi is one. There are so many others such as DigiCert, Amazon, etc.
A few years ago a non-profit initiative called Let’s Encrypt was formed with the purpose of making securing websites both easy and inexpensive. There have been many different clients created which work with Let’s Encrypt (which is a certificate authority) to issue SSL certificates simply and without cost. This has revolutionized SSL certificate issuance and usage. Anyways, that is a bit off track but some background information.
Back to Mail-in-a-Box - as a SSL certificate is required, the developers of this project have implemented one of the many Let’s Encrypt clients for the purpose of issuing the required SSL certificate. When Mail-in-a-Box is first installed, the installation procedure will automatically connect to Let’s Encrypt and request and install a SSL certificate. MOST of the time this happens flawlessly, but some times it fails. The usual reason that it fails is because of lack of propagation of DNS. There was also a time period that some changes were made within Let’s Encrypt which caused certificate issuance to fail in MiaB. I am not sure what (if any) reason existed that a LE SSL certificate was not issued for you … or if perhaps, you may have simply replaced it manually.
My personal recommendation would be to ignore the warning in the admin area, and then sometime shortly before the expiration of the Gandi certificate, replace it. The Gandi certificate is good for another 11 months. If the warning (more on the warning itself later) bothers you, then you could replace the certificate now … but again, that would not be something I would do - but you are certainly free to do so. If you are so inclined, please be certain that you are using one of the two most recent versions of MiaB. v0.29 is of course preferred.
To replace the certificate, I think that I would SSH into the VPS, and delete the contents of the /home/user-data/ssl/ directory. The rerun the command line sudo mailinabox. This will run through the installation process and when it reaches the SSL certificate part, will request and install a SSL certificate from Let’s Encrypt. There is a method to replace the SSL certificate from the admin area web-page, but I think that I would not do it that way with the possible uncertainty of any lingering issues from using the Gandi certificate.
Lastly, regarding the error message that you are seeing - that appears to me to be a bug within Mail-in-a-Box. I would suggest posting the details to the GitHub page for the project. When you look at the details of the certificate, the certificate is indeed issued for box.dobom.website. The error message is reporting that the certificate is issued for the issuer of the certificate - I do not know why, but I suspect that using a certificate from an outside provider is behind this. As MaiB provisions a certificate automatically through LE, I would imagine that this possibly has not been encountered before.