What happens when you send an email
When you click send, your message does not go directly to the recipient's inbox. Instead, it travels through at least two separate servers — one that sends it out, and one that receives it — using a system called SMTP (straightforward Mail Transfer Protocol). Your email client (Gmail, Outlook, Apple Mail, or whatever you use) connects to your provider's outgoing mail server, hands over the message, and then disconnects. The server takes it from there.
That outgoing server looks at the recipient's email address — the part after the @ sign — and finds the mail server responsible for that domain. If you send to someone at company.com, the SMTP server queries the internet's domain name system to locate company.com's mail server, then connects and delivers your message. If the recipient's server is down or busy, your server holds the message and tries again at intervals, usually for several days before giving up and bouncing it back to you.
The entire process from click to arrival typically takes seconds to a few minutes, though delays can stretch it to hours if servers are congested or if the recipient's domain is temporarily unreachable. You do not see most of this — you just see "sent" in your client.
Key Takeaways
- Email travels through two separate servers: one that sends it from your account, and one that receives it at the destination domain.
- The sender's server looks up the recipient's mail server using the domain name (the part after @) and delivers the message there.
- If the recipient's server is unreachable, your server retries automatically for several days before bouncing the message back to you.
- Attachments are encoded into the message itself and travel the same route as the text, though very large files may be rejected by either server.
- Your inbox stores messages on your provider's server, not on your computer or phone, which is why you can access it from any device.
How your inbox receives and stores messages
When a message arrives at your mail server, it does not sit in a single location. The server stores it in a database tied to your account, and your email client retrieves it using a protocol called IMAP (Internet Message Access Protocol) or the older POP3 (Post Office Protocol). The difference matters: IMAP keeps messages on the server and syncs them across all your devices, so your inbox looks the same whether you check email on your phone, laptop, or web browser. POP3 downloads messages to one device and removes them from the server, which is why you might see different messages on different devices if you use POP3.
Most modern providers use IMAP by default. When you open your email client, it connects to the server, downloads a list of message headers (the sender, subject, and date), and displays them in your inbox. The full message body and attachments stay on the server until you click to read them, at which point your client downloads just that message. This is why checking email on a slow connection can feel sluggish — your client is constantly talking to the server.
Deleting a message in your client sends a deletion command to the server, which marks the message as deleted. Most providers move it to a Trash or Deleted Items folder first, where it stays for 30 days before permanent removal. Permanently deleting it from Trash removes it from the server entirely.
Why emails sometimes take longer to arrive
A message can be delayed at several points. If your outgoing server is overloaded, it may queue your message and send it later. If the recipient's server is temporarily down or rejecting connections, your server retries — usually when ready, then after a few minutes, then after an hour, and so on, for up to five days. Network congestion between servers can also add minutes or hours.
Some delays are intentional. Many mail servers use greylisting, a spam-prevention technique that temporarily rejects unfamiliar senders and forces them to retry. The first attempt fails, the second succeeds minutes later. You do not see this — the retry happens automatically — but it can make legitimate email take 5 to 15 minutes longer than expected.
Spam filters can also hold messages. If your message triggers a filter on the recipient's server (because of the subject line, sender reputation, or content), it may be moved to their spam folder instead of their inbox. The recipient still receives it, but they may not see it unless they check spam. This is why important messages sometimes vanish.
Attachments and file size limits
When you attach a file, your email client encodes it into the message itself using a method called Base64 encoding, which makes the file about 33 percent larger. A 10 MB file becomes roughly 13 MB in the email. Most providers allow attachments up to 25 MB, though some (like Gmail) cap it at 25 MB and others at 50 MB. If your file is larger, the send will fail with an error message.
The recipient's server may also have limits. If their server rejects attachments over a certain size, your message bounces back. Some corporate email systems are stricter than consumer providers. If you need to send a large file, cloud storage services like Google Drive, Dropbox, or OneDrive let you upload the file once and share a link instead, which avoids size limits entirely.
Attachments are not scanned for viruses by your email client — that happens on the server. Both your outgoing server and the recipient's incoming server may scan attachments. If a file is flagged as malicious, the recipient's server may quarantine it, delete it, or warn the recipient before they read it.
How spam filters work and why legitimate email gets caught
Mail servers use multiple methods to identify spam. Sender reputation is one: if your domain or IP address has a history of sending spam, new messages from that address are more likely to be filtered. Content analysis is another: messages with certain words, phrases, or formatting patterns common in spam are flagged. Authentication checks
Legitimate email gets caught because it resembles spam. A marketing email with lots of links and images, or a message with urgent language like "Act now" or "Limited time", may trigger filters. Emails from new senders are more likely to be filtered than those from established contacts. If you are not receiving expected email, check your spam folder first — it is usually the fastest way to find it and mark it as legitimate, which trains the filter to allow future messages from that sender.
You can also ask the sender to check their authentication records. If their domain does not have SPF or DKIM set up correctly, their messages are more likely to be filtered, even if they are legitimate. This is a technical issue on the sender's side, not yours.
The difference between email providers and email clients
An email provider is the service that stores your messages and runs the mail servers — Gmail, Outlook, Yahoo Mail, or your company's email system. An email client is the software you use to read and send messages — Outlook (the desktop app), Apple Mail, Thunderbird, or the Gmail web interface. You can use any client with most providers, though some (like Gmail) work better with their own client.
If you use Gmail's web interface, you are using both the provider and the client at once. If you use Outlook (the desktop app) to check a Gmail account, Gmail is the provider and Outlook is the client. The client is just a window into your account on the provider's server. Your actual messages live on the provider's server, not on your computer.
This matters when you switch devices or providers. If you switch to a new email provider, your old messages stay with the old provider unless you export them. If you switch to a new client, your messages are unaffected because they are still on the provider's server. The client is replaceable; the provider is where your data lives.
Security basics: passwords, two-factor authentication, and forwarding
Your email account is the key to everything else — password resets for other services, financial accounts, and personal information all go through email. A strong password (at least 12 characters, mixing letters, numbers, and symbols) is the first line of defense. The second is two-factor authentication (2FA), which requires a second verification step (usually a code from your phone) when you log in from a new device. Most providers offer this, and turning it on takes a few minutes.
Be cautious with email forwarding. If you set up a rule to forward all incoming mail to another address, an attacker who gains access to your account can silently forward your messages to themselves while you continue to see them normally. Check your forwarding settings regularly — most providers let you view active forwarding rules in account settings. Similarly, check which apps have permission to access your email. Revoking access to apps you no longer use reduces the number of places your account can be compromised.
If you suspect your account has been compromised, change your password when ready and review your recent login activity. Most providers show a list of devices and locations that have accessed your account. If you see unfamiliar activity, sign out all sessions and enable 2FA if you have not already.
Frequently Asked Questions
Can I unsend an email after I click send?
Most providers offer an "undo send" window of 5 to 30 seconds, which delays the actual send and gives you time to cancel. Once that window closes, the message is gone from your control. Gmail, Outlook, and Yahoo all have this feature, but it only works if you catch the mistake when ready. After the window closes, the message is on the recipient's server and cannot be recalled.
Why do I see the same email multiple times?
Duplicate emails usually happen when your client syncs with the server and downloads the same message twice, or when you have the same account open in multiple clients at once. Refreshing your inbox or closing and reopening your client usually clears duplicates. If duplicates persist, check whether you have the account set up in multiple clients or whether your client is set to read all messages instead of syncing with the server.
What is the difference between CC and BCC?
CC (carbon copy) adds recipients whose addresses are visible to everyone on the message. BCC (blind carbon copy) adds recipients whose addresses are hidden from other recipients. Use CC when you want transparency — for example, copying a manager on a work email. Use BCC when you want to send the same message to multiple people without revealing their addresses to each other, or when you do not want to create a long recipient list.
How long does email take to arrive?
Most email arrives within seconds to a few minutes. Delays of an hour or more are usually caused by server congestion, greylisting, or spam filtering. If an email has not arrived after a few hours, check your spam folder, ask the sender to resend it, or contact your provider's support. Messages older than five days that have not arrived are typically lost and cannot be recovered.
Can I recover a deleted email?
Most providers keep deleted messages in a Trash or Deleted Items folder for 30 days before permanently removing them. You can usually restore a message by opening Trash, finding it, and moving it back to your inbox. After 30 days, the message is permanently deleted and cannot be recovered. Some providers offer longer retention periods for business accounts.