Email uses two separate paths. To receive, the app fetches messages from the server via IMAP or POP3. To send, the app hands messages to the server via SMTP. Because the paths differ, one can fail while the other works. That is why the inbox keeps filling while your replies sit in the outbox.
Confirm the pattern first
| Symptom | Likely cause | First step |
|---|---|---|
| Fails only on certain Wi-Fi, fine on mobile data | The network blocks the SMTP port | Switch to port 587 or 465 |
| Fails on every network | Account settings or password in the app | Sign in to the account again in the app |
| Only emails with large attachments fail | Attachment size limits or slow upload | Send a cloud storage link |
| Webmail won't open either | A general connection problem, not email-specific | Check the Wi-Fi login or the internet connection |
| A certificate warning appears when sending | The network or antivirus is inspecting encrypted connections | Don't continue; use mobile data |
| Fails since the company domain changed email providers | The outgoing server name has changed | Ask the email administrator for new settings |
If webmail won't open either, start with Wi-Fi connected but no internet or Wi-Fi login page not appearing. In hotels and cafés, email often fails simply because the Wi-Fi login page hasn't been completed.
Blocked SMTP port
Many ISPs, hotels, and office networks block port 25 to stop infected devices sending spam. An email app still using port 25 will fail on such networks. Some tightly managed networks open only web ports.
The fix is to change the outgoing server settings in the email app:
- Port 587 with STARTTLS encryption, or
- Port 465 with SSL/TLS encryption,
- and enable "outgoing server requires authentication".
Follow the numbers your email provider recommends. Other common ports are listed in ports and protocols worth knowing. If you manage an office network, allow ports 587 and 465 outbound; port 25 can stay closed.
Testing the port from a laptop
Rather than guessing, you can confirm whether the network blocks a particular port. Replace the server name below with your email provider's outgoing server.
- Windows: open PowerShell and run
Test-NetConnection smtp.gmail.com -Port 587. If the TcpTestSucceeded line says True, the port is open. If False, the port is blocked or the server is unreachable. Repeat with-Port 465. - macOS: open Terminal and run
nc -vz smtp.gmail.com 587. The word succeeded means the port is open. - Compare two networks. Run the same test with the laptop on your phone's hotspot. If the results differ, the cause is definitely that Wi-Fi network.
Where to change the outgoing server settings
- Classic Outlook on Windows: File → Account Settings → Account Settings → select the account → Change → More Settings → Advanced tab. For Microsoft 365 or Gmail accounts added through automatic sign-in, ports are usually managed for you; signing in to the account again is better than changing numbers.
- Thunderbird: Account Settings → Outgoing Server (SMTP) → select the server → Edit.
- Mail on Mac: Mail → Settings → Accounts → select the account → Server Settings → the Outgoing Mail Server section. Turn off Automatically manage connection settings to enter the port yourself.
- Mail on iPhone: Settings → Apps → Mail → Mail Accounts (on older iOS: Settings → Mail → Accounts) → select the account → Account → SMTP → primary server.
- Android: the menu differs between apps. In the Gmail app for a non-Google account: Settings → select the account → Outgoing settings.
If it fails on every network
- The password was changed but the app still uses the old one.
- Two-step verification is on. Gmail and some other services no longer accept ordinary passwords from third-party apps. Sign in again with "Sign in with Google" or Microsoft, or create an app password if the service offers one. See two-step verification.
- The account is temporarily restricted for sending too many emails in a short time. Check notices in webmail.
- The server name is wrong, for example after the company domain moved email providers.
Emails with large attachments
Most email services limit attachments to around 20 to 25 MB per message. On Wi-Fi with slow upload, large attachments can also fail midway; see why upload is slower than download. For large files, upload them to cloud storage and send the link.
When it is not your device
If the port test fails on one Wi-Fi and succeeds on another network, there is nothing to fix on the laptop. Your options:
- Use webmail in the browser while on that network.
- Use mobile data or your phone's hotspot briefly to send the stuck messages.
- Use a trusted VPN, which wraps all traffic so the network can't see the SMTP port. If the VPN makes sending fail instead, the email provider may be blocking the VPN server. See VPN for public Wi-Fi.
- Ask the network administrator at the office or hotel. They can open ports 587 and 465 without opening port 25.
Other causes
- Antivirus or firewall on the laptop that scans outgoing mail sometimes interferes with encrypted SMTP connections. Try temporarily turning off email scanning.
- DNS problems stop the mail server name from resolving; see DNS server not responding.
- A certificate warning when sending on public Wi-Fi may mean someone is trying to read the connection. Don't accept an unknown certificate; send over mobile data instead.
Common myths
- "Changing DNS will fix it." DNS only translates server names. If the port is blocked, changing DNS doesn't help.
- "Turn off encryption so email can get out." Sending without encryption lets passwords and message content be read on public networks, and many email providers already refuse it.
- "Stuck email means the account was hacked." It is almost always a port or settings issue. Check account activity in webmail if you are unsure.
The "one service fails, others work" pattern is covered more generally in certain apps won't open.
Frequently asked questions
Why can I receive email but not send it?
Because receiving and sending use different paths. Receiving uses IMAP or POP3, while sending uses SMTP. If the network blocks the SMTP port the app uses, email still comes in but can't go out.
Which SMTP port should I use?
For email apps on laptops and phones, use port 587 with STARTTLS or port 465 with SSL/TLS, as your email provider advises. Port 25 is used between mail servers and is often blocked on user networks, so avoid it for apps.
Why does webmail send but the email app doesn't?
Webmail runs over HTTPS on port 443, which is almost always open. Email apps use SMTP on other ports that a network may block. Until it's fixed, send important email through webmail or mobile data.
How can I check whether the SMTP port is blocked on my network?
On Windows, open PowerShell and run Test-NetConnection followed by the outgoing server name and -Port 587, for example Test-NetConnection smtp.gmail.com -Port 587. If TcpTestSucceeded is False on that Wi-Fi but True on another network, the port is blocked. On macOS, nc -vz smtp.gmail.com 587 in Terminal gives a similar answer.
Will email stuck in the outbox send automatically later?
Usually, yes. The email app retries once it can reach the outgoing server, for example after you change networks. Some apps stop retrying after several failures, though. Check the outbox after switching networks, and resend manually if the message is still stuck.
