Thanks for the quick update. From experience even when we had couple of thousands users there were less than 300 outgoing messages daily but what I do notice is virtually all users that login daily would connect multiple times daily.

Based on that analysis what sort of linode plan do you think will be sufficient per month?
https://www.linode.com/pricing

Also is the user mailbox size /dependent/configured to the RAM e.g 2GB or the Server Storage e.g from 24SSD size?

NOTE: I am still expecting your update to the email I responded to in the morning "Migration request"

I like Digital Ocean, very much, but had multiple bad experiences running mail servers there. The issue is, at least for DO's California POP, that the IP addresses in use are known spam sources, and large providers such as AOL, Microsoft, and Yahoo will block them. There are ways to get off these blacklists, but it's an extra step, and it doesn't always work -- in my case, AOL never would lift the blacklist because it continued to get spam from the same subnet, but not from my servers. So, basically, it was a bad neighborhood for an email server.

I use RootBSD instead, and it's been foolproof. It's a little more expensive but I have not had the same issues with blacklisting.

Again, I like Digital Ocean, but it just wasn't a good place for an email server.