Chilkat version 9.3.0, being released this week, includes new IMAP functionality to fetch emails without downloading attachments, and then subsequently fetch attachments one at a time separately.

There is a new boolean property named “AutoDownloadAttachments”, which has  a default value of True (YES).  If set to False (NO), then all Fetch* methods will not download attachments.

Note:  “related” items are downloaded.  These are images, style sheets, etc. that are embedded within the
HTML body of an email, and are not considered attachments.

Also:  All signed and/or encrypted emails must be downloaded in full.

When an email is downloaded without attachments, the attachment information is included in header fields.  The header fields have names beginning with “ckx-imap-“.  The attachment information can be obtained via the following methods:

imap.GetMailNumAttach
imap.GetMailAttachFilename
imap.GetMailAttachSize

Individual attachments can be downloaded via the newly added FetchAttachment* methods:

1. FetchAttachment:  Downloads an attachment and saves to a file.
2. FetchAttachmentBytes:  Downloads an attachment to an in-memory byte array.
3. FetchAttachmentString: Downloads an attachment to a string variable.

Here are the FetchAttachment* method signatures in Objective-C:

// method: FetchAttachment
– (BOOL)FetchAttachment: (CkoEmail *)email
attachIndex: (NSNumber *)attachIndex
saveToPath: (NSString *)saveToPath;

// method: FetchAttachmentBytes
– (NSData *)FetchAttachmentBytes: (CkoEmail *)email
attachIndex: (NSNumber *)attachIndex;

// method: FetchAttachmentString
– (NSString *)FetchAttachmentString: (CkoEmail *)email
attachIndex: (NSNumber *)attachIndex
charset: (NSString *)charset;

Question:

When using IMAP and fetching emails by sequence, does looping through the sequence get affected by deleting in mid-sequence or other changes to the mailbox’s content on the server?

Answer:

The answer is obtained by examining the IMAP specification (RFC 3501).  Here’s what it says:

2.3.1.2.        Message Sequence Number Message Attribute

   A relative position from 1 to the number of messages in the mailbox.
   This position MUST be ordered by ascending unique identifier.  As
   each new message is added, it is assigned a message sequence number
   that is 1 higher than the number of messages in the mailbox before
   that new message was added.

   Message sequence numbers can be reassigned during the session.  For
   example, when a message is permanently removed (expunged) from the
   mailbox, the message sequence number for all subsequent messages is
   decremented.  The number of messages in the mailbox is also
   decremented.  Similarly, a new message can be assigned a message
   sequence number that was once held by some other message prior to an
   expunge.

   In addition to accessing messages by relative position in the
   mailbox, message sequence numbers can be used in mathematical
   calculations.  For example, if an untagged "11 EXISTS" is received,
   and previously an untagged "8 EXISTS" was received, three new
   messages have arrived with message sequence numbers of 9, 10, and 11.
   Another example, if message 287 in a 523 message mailbox has UID
   12345, there are exactly 286 messages which have lesser UIDs and 236
   messages which have greater UIDs.