When you set up a mailbox in a phone or Outlook, it asks whether to use IMAP or POP3. They are two ways of getting mail off the server and into your client, and they behave very differently. The answer for almost everyone is IMAP; here is why, and when the exception applies.
POP3: download and (usually) delete
POP3 — Post Office Protocol, version 3 — dates from 1988. The client connects, downloads any new messages to the device, and by default deletes them from the server. Mail then exists in one place: that device. Folders, read/unread state and sent items are local to the client.
It was designed for a world of one computer per person and expensive dial-up: connect briefly, take the mail, disconnect.
IMAP: the mailbox lives on the server
IMAP — Internet Message Access Protocol — keeps the mailbox on the server and lets clients view and manipulate it. Every device sees the same folders, the same read/unread flags, the same sent items. Delete a message on your phone and it is gone from Outlook too. Messages are downloaded on demand (or cached), so the server is the source of truth.
This is how Gmail, Outlook.com and every hosted mailbox behave, and what people now expect email to be.
Side by side
| IMAP | POP3 | |
|---|---|---|
| Mail stored | on the server | on one device |
| Several devices | all in sync | each downloads separately, or only the first gets it |
| Folders and flags | synced | local only |
| Sent items | synced | local only |
| Server storage used | the whole mailbox | almost none (if deleting) |
| Works offline | cached copies, full sync when online | fully |
| Backup | the server's backup covers it | the device's backup, if any |
| Ports | 143 (STARTTLS) / 993 (TLS) | 110 (STARTTLS) / 995 (TLS) |
Choose IMAP when
Always, unless one of the cases below applies. Phones, laptops, webmail and desktop clients all stay consistent, the server's backups protect the mail, and a lost laptop does not mean lost email.
The cost is server storage: a mailbox that has accumulated ten years of attachments is 20 GB on the server. Set quotas in Plesk, archive old mail locally when needed, and it is a non-issue. Creating mailboxes with quotas is covered in creating email accounts in Plesk.
Choose POP3 when
- A single machine collects mail into a local archive — an accounting system that ingests invoices, or a person who genuinely uses one PC and wants nothing on the server.
- Mailbox storage is very limited and cannot be increased.
- Regulatory or preference reasons to hold mail only on your own hardware.
If you do use POP3, set the client to leave a copy on the server for at least a few days so a second device or webmail can still see recent mail, and so a client crash mid-download does not lose messages.
The mixed setup that goes wrong
One person on IMAP on their phone and POP3 in Outlook on the desktop. Outlook downloads and deletes; the phone then shows an empty inbox and the person reports "my mail is vanishing". If you inherit a setup where mail disappears from one device after being seen on another, that is what is happening; switch the POP3 client to IMAP.
Settings for a Plesk mailbox
Plesk shows these when you click a mailbox, and they are the same on every VPSPioneer plan:
- IMAP:
mail.example.com, port 993, SSL/TLS, username = full address. - SMTP (sending):
mail.example.com, port 465 SSL/TLS or 587 STARTTLS, same username and password. - POP3, if you must: port 995, SSL/TLS.
Step-by-step for the common clients: setting up Plesk email on iPhone, Outlook and Gmail.