On the mailbox screen, you can quickly search for messages whose 'subject' or 'from' address contains a particular string by entering the string into the search text box and clicking 'search'. At this time you cannot search in more than one folder at once.

To perform an advanced search, leave the text box blank, and click the 'Advanced' button, this will take you to the advanced search screen. Enter your search criteria in the fields provided. Click the 'Search' button to return to the Mailbox screen and perform the search.

Enter the search criteria you want on the Advanced search screen, then enter a name you want to give the search in the "Save search" section. Click the "Save" button to store the search. You can then access this saved search from the Advanced screen in the future, or you can access it quickly from the "Action" menu at the top left of the Mailbox screen

A value in "Custom" is passed directly to the IMAP server. See the IMAP specification for full details. Here is a quick reference of the commands (thanks to forum user Sanjay Sheth for compiling this):

ALL

All messages in the mailbox; the default initial key for ANDing.

ANSWERED

Messages with the \Answered flag set.

BCC <string>

Messages that contain the specified string in the envelope structure's BCC field.

BEFORE <date>

Messages whose internal date is earlier than the specified date.

BODY <string>

Messages that contain the specified string in the body of the message.

CC <string>

Messages that contain the specified string in the envelope structure's CC field.

DELETED

Messages with the \Deleted flag set.

DRAFT

Messages with the \Draft flag set.

FLAGGED

Messages with the \Flagged flag set.

FROM <string>

Messages that contain the specified string in the envelope structure's FROM field.

HEADER <field-name> <string>

Messages that have a header with the specified field-name and that contains the specified string in the field-body.

KEYWORD <flag>

Messages with the specified keyword set.

LARGER <n>

Messages with an size larger than the specified number of octets.

NEW

Messages that have the \Recent flag set but not the \Seen flag. This is functionally equivalent to "(RECENT UNSEEN)".

NOT <search-key>

Messages that do not match the specified search key. Eg: NOT BODY "cat"

OLD

Messages that do not have the \Recent flag set. This is functionally equivalent to "NOT RECENT" (as opposed to "NOT NEW").

ON <date>

Messages whose internal date is within the specified date.

OR <search-key1> <search-key2>

Messages that match either search key. Eg: OR BODY "cat" BODY "dog"

RECENT

Messages that have the \Recent flag set.

SEEN

Messages that have the \Seen flag set.

SENTBEFORE <date>

Messages whose Date: header is earlier than the specified date.

SENTON <date>

Messages whose Date: header is within the specified date.

SENTSINCE <date>

Messages whose Date: header is within or later than the specified date.

SINCE <date>

Messages whose internal date is within or later than the specified date.

SMALLER <n>

Messages with an size smaller than the specified number of octets.

SUBJECT <string>

Messages that contain the specified string in the envelope structure's SUBJECT field.

TEXT <string>

Messages that contain the specified string in the header or body of the message.

TO <string>

Messages that contain the specified string in the envelope structure's TO field.

UID <message set>

Messages with unique identifiers corresponding to the specified unique identifier set.

UNANSWERED

Messages that do not have the \Answered flag set.

UNDELETED

Messages that do not have the \Deleted flag set.

UNDRAFT

Messages that do not have the \Draft flag set.

UNFLAGGED

Messages that do not have the \Flagged flag set.

UNKEYWORD <flag>

Messages that do not have the specified keyword set.

UNSEEN

Messages that do not have the \Seen flag set.