Could run https://poste.io locally?

IMO, you're overcomplicating things a lot. You already have a working mail server, which is not easy to setup and maintain. Adding another mail server and redistributing mail storage / routing mail between the two will drastically decrease stability and may affect deliverability of your mail as well.

Why not just add storage to the existing VPS? If your hosting provider doesn't provide such option, you can mount third-party S3-compatible storage (rclone mount works great, a few hundred Gb on Backblaze cost virtually nothing) and store your mail there.

You could do what I did and move the mail server to a local machine or VM then use Wireguard on the VPS which tells iptables to route incoming mail traffic ports to the now local mail server. Then you can use as much space as you want.

Are you looking for a way to archive older mails like Mailstore ?

I use fetchmail to grab mail from an email provider and deliver it to my dovecot instance hosted at home. My mail provider is just used to store mail temporarly (up to a minute unless my server is down) and send email (I use their SMTP server).

That way, my data is stored at home but I don't have usual 'selfhosted mail server' issues. I can be down for days and my email will be received by my provider and I don't need to do/setup/maintain anything to ensure my mail will be deliverable!

With a custom domain and nothing that stay on their server, I can switch provider within a few minutes by updating the nameserver to point my domain to another server

To manage large email accounts on Xplornet without the need for additional server space for old emails (20GB+), you can set up a self-hosted IMAP archiving solution, like Dovecot, on your own server. This allows you to archive older emails locally while still using Xplornet's webmail system for sending and receiving messages. If multiple users need access, you can deploy a webmail solution like Roundcube or Rainloop. This setup enables you and your family to access archived emails and compose new ones using their SMTP credentials, similar to how Gmail or Outlook handle POP3 accounts. This way, you can maintain email accounts on Xplornet while archiving emails efficiently.