When it comes to sending reminders for follow-ups, approvals, and payments, channel choice matters more than most people realize. The wrong channel can mean the difference between a same-day response and a week of silence.

We do not have a dataset to settle this, and neither does anyone else selling you a tool. What follows is the reasoning, so you can judge it against your own situation rather than against a number somebody made up.

The Short Answer

For internal teams: Slack, because a teammate already has it open and a one line reply costs them nothing.

For external clients/vendors: Email wins, more professional, creates a paper trail, and clients check it more.

But the nuance matters. Let's break it down.

What Actually Decides It

Not response time in the abstract. Three things about the person you are chasing.

Do they already have the app open? A teammate has Slack open all day, so a message there is seen in minutes. A client does not have your Slack at all, so the same message reaches nobody.

Does the request need a record? An invoice chase, a scope change and a deadline commitment all benefit from sitting in a thread both parties can find in six months. Slack history is searchable, but it is your workspace, not theirs.

How formal is the relationship? A short "any update on this?" reads as friendly from a colleague and as curt from a supplier you have met twice.

Why Slack Wins Internally

Lower friction

Slack feels conversational. Replies are short, fast, and don't require formality. A quick "On it!" is enough.

Presence indicators

You can see when someone is online, so a nudge can land while they are at their desk rather than at the bottom of a pile they read tomorrow.

Channel context

In Slack, reminders in a project channel are visible to the whole team, creating gentle social accountability.

Why Email Wins Externally

Professionalism signal

Email still carries more professional weight for business transactions, particularly payments and formal approvals.

Client habits

Most clients check email more reliably than any other communication tool, it's universal.

Documentation

Email creates a clean paper trail that's valuable for disputes, audits, and compliance.

Use Cases: Which Channel to Choose

Use Slack for:

Internal project deadlines
Team task approvals
Stand-up reminders
Cross-functional coordination
Anything time-sensitive with internal stakeholders

Use Email for:

Invoice reminders
Client follow-ups
External vendor communications
Formal notices
Legal or compliance-related follow-ups

The Hybrid Approach

The most effective teams use both strategically. Here's the pattern that gets the best results:

Initial outreach: Email (formal record)
First follow-up: Email (same thread)
Second follow-up: Slack DM if available ("Hey, did you see my email about X?")
Third follow-up: Email with urgency escalation

This hybrid approach combines email's professionalism with Slack's immediacy.

Setting Up Multi-Channel Reminders in autoremind.ai

autoremind.ai sends on both channels, one channel per reminder:

Email for clients, suppliers and anyone outside your company. It sends from your own Gmail in the original thread and stops on its own when they reply, using a tracked reply address rather than reading your inbox. This is the main channel, and the one most people use.
Slack for teammates. It posts to their DM or a channel from your own Slack account. It never reads Slack, so it cannot see a Slack reply; when they answer, you close the reminder.

A reminder does not switch channel halfway through. To run the hybrid pattern above, start with an email reminder, and if it goes unanswered and the person is in your Slack, add a short Slack reminder pointing at the email. Each one is a sentence to set up:

"Follow up with Dana about approving the project scope I emailed last Tuesday."

"Nudge Dana on Slack to check my email about the project scope."

If you are looking for Slack's own automation (scheduled messages, /remind, Workflow Builder), Slack automated messages: every way to set them up walks through each one.

Conclusion

There's no one-size-fits-all answer, and that's exactly the point. The best channel is the one your recipient uses most.

For remote-first teams, that's Slack. For external business relationships, that's email. The best reminder systems use both.

The important thing is to actually send the reminders. Most people don't follow up consistently enough, and that's the single biggest factor in whether you get responses.

Slack vs Email for Business Reminders: Which Works Better?