Is there a MAX number of emails that can reside indefinitely in a folder? Specifically, INBOX, but can move emails into another folder as well. I have about 7500 in my inbox. I like to keep emails forever, just in case.
The only limit is your hard drive.
While Josh is (as always!) succinct and correct, if you’d like numbers: I have 5 domains housing thousands of emails spanning something like 5 years, and using 6-7GB of storage. I have had no issues whatsoever. Others here have had up to fifty users in company environments using MIAB on terabyte scale physical servers with no issues at all. I plan to eventually merge my historical email archive into my MIAB, which is about 300,000 emails at ~27GB, and I have no reservations about performance.
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ALSO, this is obvious but might be overlooked by someone.
MIAB server and checking email via roundcube may be essentially limitless, but we often check through email client on laptop, desktop and, even more limited, cell phone.
IMAP still takes up space on harddrive, which is dependent on how long you have messages synced. I used to go with sync ALL messages, until brand new 2TB SSD MAC Laptop had no space left . Even worse, new samsung note 5 would crawl to a halt. So, keep this in mind for folders with thousands of emails.