IMAP

There are two dominant protocols on the Internet for reading/downloading email from servers (at least if we do not count web based reading), and they are IMAP and POP3. The former is the slightly more modern alternative. curl supports both. IMAP also allows emails to get uploaded, which POP3 does not.

Basics

Get the mail using the UID 57 from mailbox ‘stuff’:

curl imap://server.example.com/stuff;UID=57

Instead, get the mail with index 57 from the mailbox ‘fun’:

curl imap://server.example.com/fun;MAILINDEX=57

List the mails in the mailbox ‘boring’:

curl imap://server.example.com/boring

List the mails in the mailbox ‘boring’ and provide user and password:

curl imap://server.example.com/boring -u user:password

TLS for IMAP

IMAP can be done over a secure connection and it can be done using either explicit or implicit TLS. The explicit method is probably the most common approach and it means that the client connects to the server using an insecure connection and upgrades it to TLS as it goes, using the STARTTLS command.

You tell curl to use this upgrade approach with --ssl-reqd. It says that the upgrade is required to work or curl fails the transfer. There is also the not-recommended insecure alternative --ssl that attempts to use TLS but that continues even if the upgrade fails.

Example:

curl --ssl-reqd imap://server.example.com/boring

Implicit SSL means that the connection gets secured already at first connect, which you make curl attempt by specifying a scheme in the URL that uses SSL. In the case of secure IMAP that means imaps://. For such connections, curl insists on connecting and negotiating a TLS connection already from the start, or it fails its operation.

The previous explicit examples done with implicit SSL:

curl imaps://mail.example.com/inbox

Upload

Uploading data to an IMAP server means putting an email into a specific remote “mailbox”.

curl imap://imap.example/mailbox -T email.txt -u user:secret \
  --ssl-reqd

When curl uploads an email to an IMAP mailbox, it will by default flag that email as already read. In IMAP terms, it is marked as seen.

You can alter the default flags for IMAP uploads using the --upload-flags option. Make the newly uploaded email appear as answered, deleted, draft, flagged or seen by providing your set in a comma-separated list.

Negate a flag by prefixing it with a minus (-).

For example, mark the upload as not seen and a draft:

curl imap://imap.example/mailbox -T email.txt \
  --upload-flags "-seen,draft"