Googling 'dovecot high volume server' leads to this discussion from 2015:
https://dovecot.org/pipermail/dovecot/2015-September/102017.htmlIn that thread there is discussion of dovecot installs with 28K total users and up to 4K mailboxes per server, with some server hardware listed.
I couldn't find anything else talking about the specific requirements for sizing a high volume server.
* You would likely need to adjust some of the SME default settings.
* You might want to have high performance storage (ssd-based or with an ssd cache)
* If possible, you may want to gather some specs on the current system if there is one
- new emails received per day
- emails sent per day
- average email size
- average mailbox size
- pop3, pop3s, smtp, and smtps traffic by hour/day/week
...and a warning:
I had an office many years ago where all users used POP3 but set their client to never delete mail from the server. Everything worked fine until a user's mailbox reached a specific size, then bad things would happen. When the time required for the client to scan the mailbox exceeded the time between pop3 mailbox checks it started taking 2 to 4 hours for new mail to be seen by the pop3 client. The "solution" for these users was to switch them to IMAP...