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So back to my original problem now - I'm thinking of either putting procmail on Synology and fetch emails or either use iptables/VPN route to send email home.

If @binary1zero is using some business IP from their ISP, one can just host the mail server from home/office instead of all this forwarding.

From what I know, PTR and stuff is only necessary for outbound relay

that's correct. PTR is for the server that does the mail transfer, so it won't be needed if an external relay is used. On Cloudron, the PTR check is skipped when using a relay.

From what I know, PTR and stuff is only necessary for outbound relay, as it mostly affects server reputation when other server decide incoming email is spam.

Outbound is answered via that post.

For inbound, here's what I'm thinking now :

• Setup private wireguard connection between DO and my home server
• Turn off Inbound email on DO Cloudron.
• Redirect inbound ports like TCP/25 via iptables on DO to home server via wireguard

https://forum.cloudron.io/topic/6046/proxy-email-via-vps/9

EDIT : could a sieve filter work to forward all incoming email to another server ? Then just delete/purge periodically on initial receiving server after a time period.

I've looked into this but it doesn't seems to be an option. Sieve filter can only redirect to another "email" rather than a mail server. In this case - both emails would be same as we are just forwarding it to a remote mailbox.

@binary1zero there is no built-in solution for forwarding to another email server.

This is indeed an interesting request though. I guess this allows a setup where the mail is actually stored at home? Also, if the server is at home, how will you access email when outside? Does this mean you will have the home server partially exposed to the world for mail access? In addition, on many ISPs, inbound port 25 is blocked (yes, even inbound, like in comcast).

Exactly what you said. Plan is to run Cloudron/or other mail server on my Synology at home which has ample amount of storage but since ISP cannot give PTR and other settings - I'm using DigitalOcean droplet with Cloudron for my incoming mail (and it works beautifully).

However, it's expensive to store big mailboxes in cloud. So idea is to receive email via cloudron but forward to downstream mail server at home. Home mail server will be exposed enough to access via IMAP etc.. and it will relay all outbound emails via Droplet/Cloud.

• MailPiler (custom package available for Cloudron)
• MailSteward (proprietary paid but affordable and solid local archiving solution)
• imapfilter (https://github.com/lefcha/imapfilter) to suck out emails and move them to storage location
• offlineimap (http://www.offlineimap.org/ and https://www.dermitch.de/post/imap-backup-with-offlineimap/)
• various "server-suck-file-save" scripts

Maybe I am missing something in your plan but these could be simpler to implement.

EDIT : could a sieve filter work to forward all incoming email to another server ? Then just delete/purge periodically on initial receiving server after a time period.

The more robust approach may be to have mail backups be exported to a private Minio instance or similar. (also set up mail pruning on the DO side after some size limit)

@binary1zero
Can you elaborate more on this use case, as there may be a more effective way of accomplishing the need, without using SMTP for example which isn't optimal for something like this.

Emails are relatively small, so if storage is an issue, that may be an overloaded use of the protocol for which better options exist.

This is indeed an interesting request though. I guess this allows a setup where the mail is actually stored at home? Also, if the server is at home, how will you access email when outside? Does this mean you will have the home server partially exposed to the world for mail access? In addition, on many ISPs, inbound port 25 is blocked (yes, even inbound, like in comcast).