Use Slack (or Teams) for anything that's quick, informal, and time-sensitive within your team — a question you need answered in the next hour, a heads-up, a "can you jump on a call." Use email for anything that needs a durable record, involves someone outside your team, or doesn't require an immediate response. The line is asynchronous vs. synchronous, not "which app do I have open."
I once watched a project nearly derail because a client's sign-off lived in a Slack thread that got archived when someone reorganized channels. The decision was real. The proof of it wasn't. Meanwhile, I've sent "lunch?" as an email and gotten a reply four hours later, by which point I'd already eaten a sad desk salad alone.
Both of those are the same mistake in opposite directions: using the wrong channel for the message. It's a small thing that quietly costs teams hours a week in confusion, re-asking, and things that fall through cracks nobody knew existed.
Why "just use both and hope" doesn't work
Most teams don't have a channel problem, they have a no-decision problem. The default is whichever tool is open, which means the same type of message ends up in both places depending on who's sending it and how busy they are.
Some teams try to fix this with a blanket rule — "everything internal goes in Slack." It feels clean for about two weeks. Then someone needs to reference a decision from six weeks ago and it's buried in a channel with 4,000 messages and no search term that finds it.
Others try "email for anything official," which sounds responsible but quietly kills momentum. Quick questions turn into formal-feeling threads with subject lines, and people start avoiding email altogether because it feels heavier than the question deserves.
The problem with both fixes is they're channel-based rules, not message-based ones. The channel should follow the nature of the message — not the other way around.
The system: sort by three questions, not by habit
Before sending anything, ask three quick questions. This takes less time than it does to read this sentence, and it gets automatic fast.
1. Does this need to be findable in three months? If yes, it's email. Contracts, approvals, anything with a deadline or a commitment, anything involving a client or vendor — these need a searchable, exportable record that isn't dependent on your team still using the same chat tool next year. Slack search is fine for last week. It's bad for institutional memory.
2. Does this involve someone outside your immediate team? If the recipient isn't in your Slack workspace — a client, a contractor, another department on a different tool — it's email by default. Don't invite external people into Slack just to avoid writing an email; it creates channel sprawl and access-control headaches nobody wants to manage six months from now.
3. Does this need an answer in minutes, or is a same-day response fine? Minutes-level urgency goes to Slack, with a direct @mention, not a channel post you hope someone sees. Same-day-or-later goes to email, where it won't get buried under memes and standup notes.
Here's the concrete setup: create a shared team doc (one page, not a wiki maze) with these three questions and 3–4 examples specific to your work — "client deliverable feedback → email," "is the deploy done → Slack," "quarterly planning notes → email," "can someone review this PR → Slack." Pin it in your team's main Slack channel. Revisit it once when it stops matching reality, not on a recurring schedule nobody follows.
One more habit worth adding: if a Slack thread produces a decision, someone recaps it in an email or a shared doc before the day ends. The conversation can stay fast. The outcome still needs a home that outlives the scroll.
Where this needs adjusting
If you're in a customer-facing role with an SLA, the "same-day is fine" rule doesn't apply — check what your team's actual response-time commitment is and build around that instead. If your company doesn't use Slack or Teams at all, this becomes moot; some teams genuinely run better on email alone, and that's a legitimate choice, not a failure to modernize.
Also worth naming: this only works if your team agrees to it together. If half your teammates keep defaulting to whichever tool is open, the system quietly falls apart and you're back to guessing. It's worth a five-minute conversation in your next team meeting rather than assuming everyone will read the doc.
One thing to do today
Don't build the whole system this afternoon. Just do this: the next time you're about to send a message, pause for two seconds and ask "does this need to be found later, or does this need an answer right now?" Route it accordingly. That's the entire rule, distilled.
And if email is where the important, record-worthy stuff piles up, it helps to have something sorting it for you instead of you sorting it at 9pm. That's what Email Triage actually does — it reads what's landed in your inbox, drafts replies for the routine stuff, and flags what genuinely needs your attention so email stops being the thing you're afraid to open. Try Email Triage Free if that sounds like relief.