IMAP Connection Calculator

Email servers don’t just hang around waiting for mail to arrive. No, they’re busy things. They is busy entrances, always opening and closing. Someone’s always at that digital door, your phone waking up from sleep, a new tab in your browser opening, some script checking for updates. Too many people knocking all at once though, and the whole thing jams. Nobody gets any mail.

In most situations it isn’t about how much space there is but rather how many IMAP connections is being made. You plug in how many clients you have and their usage patterns; the calculator do the rest. No more guesswork based off conversion rates or coefficients. No more fuzzy ideas of what your servers can handles. It turns vague “it feels slow” into hard data such as login attempt rate per minute and concurrent session counts.

How to Plan Your Email Server Connections

That’s important because different resources is stressed by short spikes vs. Some connections stay active for a long time. Leaving your laptop on all day stresses one set of resources, while your phone constantly dropping out of WiFi range stress another. If you want consistent performance, you should of plan for both scenario.

But first think about the number of devices per user. How many do they really have? Chances are it’s not only one. Most users access their email on multiple device: on their work laptop, in addition to their phone, maybe even on a tablet or an old computer at the office. Two client per user may underestimate things. More likely three or four clients per user are a more reasonable estimate in today’s moddern workplace, which blends corporate and personal devices.

You can set that multiplier yourself and play around with results. It tends to surprise folks how quickly those expected connections rise once you give up the assumption of one device per user.

But that’s not all, what do your clients do? Do they use IDLE to leave their connections open so that incoming mail will notify them instantly? Or do they disconnect and then reconnect periodically to see if anything changed? Sounds good, but imagine 50 users doing this every minute: an endless flood of authentication requests. The calculator accounts for such behavior; IDLE sessions gobble up file descriptors and memory; polling spikes causes stress on the authen workers.

For many admins, one is optimized at the expense of the other (which kills the server). But there’s one more thing to consider: bursts. These can exceeds your normal averages by three or four times. Whether it’s a migration project, mass password reset or people logging in during the morning rush, they’re 3-4x higher then what we’d call “normal.” The tool has built-in burst multipliers so that you can size up for those moments, not just quiet days. Your server may be able to handle Tuesday, but if you don’t leave headroom it could fail on Monday morning.

Then there’s the added layer of load balancers/proxy pools. Ideally, traffic gets distributed evenly across all your front-end nodes. Sometimes it doesn’t distribute evenly. Uneven routing or sticky sessions will dump half the reconnect storm on one machine while others sits idle. Typical profiles for these scenarios are laid out in the reference tables. These help you understand what your setup look like: Is this a desktop focused office suite? Or is this a mobile heavy environment?

Changing the safety buffer percentage provides a cushion from uneven spikes. That’s what real world tuning is: not really about a specific #, but understanding the break points. Pay attention to auth timeout settings. Long logins from slow ldap or 2fa can holds open a slot that holds up other people trying to login in a storm. Freeing them clears space for people who need it. It does because it does. It doesn’t care about maxing connections, it just wants the front door to stay clear so no one gets locked out.

Prepare for the peak, plan for chaos, and leave some room for unexpected connection.