diff --git a/README.md b/README.md
index 638fcc9..ddcd09b 100644
--- a/README.md
+++ b/README.md
@@ -7,6 +7,8 @@
 Supporting four different mbox file formats in pure golang.
 
 Package mbox implements a reader and writer for working with mbox files.
+It also provides a tool to potentially determine the type of mbox format,
+although it isn't possible to create tool that can definitively determine this.
 
 The package supports four types of mbox files:
 
@@ -29,7 +31,10 @@ Use `mboxcl2` to address the lines starting with 'From ' by doing what
 mboxcl does, except it doesn't add '>' characters at all.
 
 You may need to know which type to use when reading or writing an mbox, for
-best results.
+best results.  However, you can try using `DetectType()` to work out the type
+of mbox.  Thanks go to [BenjamenMeyer's Thunderbird Mailbox Deduper
+code](https://github.com/BenjamenMeyer/go-tb-dedup) for incentivizing me to
+create `DetectType()`, even if I took a different approach.
 
 NOTE: These routines do not concern themselves with file locking. You may want
 to consider that while working with mbox files on systems that might actively
diff --git a/detect.go b/detect.go
new file mode 100644
index 0000000..d5bc712
--- /dev/null
+++ b/detect.go
@@ -0,0 +1,145 @@
+package mbox
+
+import (
+	"bufio"
+	"fmt"
+	"io"
+	"regexp"
+	"strconv"
+	"strings"
+)
+
+func lineFeedType(reader io.ReadSeeker) (bool, error) {
+	_, err := reader.Seek(0, io.SeekStart)
+	if err != nil {
+		return false, err
+	}
+	b := make([]byte, 1024)
+	for count, err := reader.Read(b); err == nil; {
+		for i := 0; i < count; i++ {
+			if b[i] == '\r' {
+				return true, nil
+			}
+			if b[i] == '\n' {
+				return false, nil
+			}
+		}
+	}
+	return false, fmt.Errorf("no carriage return or line feed")
+}
+
+// DetectType attempts to figure out the type of mbox the reader holds.  This
+// is a best-effort attempt to determine the type of mbox file format based on
+// what it sees within the text.
+//
+// It tries to work out the type of file by:
+//   - Looking for 'Content-Length' in a message's header
+//   - Looking for '>From ' or '>>From ' (or any number of > character in front
+//     of "From "') in the message's body.
+//   - Using the length of the 'Content-Length', if present, to determine when
+//     the body of the message is complete.
+//
+// With this information, it can guess if the mbox matches one of the file
+// types supported by this library with some degree of certainty.
+func DetectType(reader io.ReadSeeker) (mboxType int, err error) {
+	rdMatch := regexp.MustCompile(`^>*>From `)
+	clMatch := regexp.MustCompile(`^Content-Length:`)
+	feedType, err := lineFeedType(reader)
+	if err != nil {
+		return -1, err
+	}
+	_, err = reader.Seek(0, io.SeekStart)
+	if err != nil {
+		return -1, err
+	}
+	inHeader := false
+	scanner := bufio.NewScanner(reader)
+	var hasRd bool = false
+	var hasCL bool = false
+	var count int64 = 0
+	var clLen int64 = 0
+	var finishedFirst bool = false
+	for scanner.Scan() {
+		// NOTE:
+		// The man page for mbox indicates that one can tell different mailings
+		// apart via lines that start with From followed by a space.  It also
+		// states that the message is RFC 822 encoded.  An RFC 822 encoded text
+		// message separates the header from the body via a 'null' line (CRLF)
+		// Ergo, for our purposes, we'll scan over the file, assuming From_
+		// indicates the start of the header, and a null line indicates the
+		// start of the body.
+
+		// Trimming the space to ensure extranous characters won't figure into
+		// this.
+		line := strings.TrimSpace(scanner.Text())
+		if inHeader && len(line) == 0 {
+			inHeader = false
+			count = 0
+			finishedFirst = true
+			continue
+		}
+		if !inHeader && !hasCL && strings.HasPrefix(line, "From ") {
+			inHeader = true
+		}
+		if !inHeader && hasCL {
+			count += int64(len(scanner.Text())) + 1
+			if feedType {
+				count += 1
+			}
+		}
+
+		matchRd := rdMatch.MatchString(line)
+		matchCL := clMatch.MatchString(line)
+
+		if inHeader && matchCL {
+			hasCL = true
+			// We have a content-length.  We need to parse it to determine the
+			// length of the body.
+			sp := strings.Split(line, ":")
+			if len(sp) < 2 {
+				// well, er, this is awkward...
+				continue
+			}
+			intCL, err := strconv.ParseInt(strings.TrimSpace(sp[1]), 10, 64)
+			if err != nil {
+				// I guess that wasn't an int.
+				continue
+			}
+			clLen = intCL
+		}
+
+		if !inHeader && matchRd {
+			hasRd = true
+		}
+
+		if hasRd && hasCL {
+			// We have enough evidence:
+			// This has content length & >From_ in the body.
+			return MBOXCL, nil
+		}
+		if hasCL && !inHeader && strings.HasPrefix(line, "From ") {
+			// We have enough evidence:
+			// This has content length, we're in the body, and the line starts
+			// with 'From '.
+			return MBOXCL2, nil
+		}
+		if !inHeader && clLen == count {
+			count = 0
+			finishedFirst = true
+			// This isn't technically true, but helps for our tests.
+			inHeader = true
+		}
+
+		if finishedFirst && !hasCL && hasRd {
+			// We have enough evidence:
+			// This doesn't have content length, but does have lines in the
+			// body starting with >First.
+			return MBOXRD, nil
+		}
+	}
+	if hasCL && !hasRd {
+		// We don't really know.  It could be MBOXCL2, or MBOXCL.  We will err on the side of caution.
+		return MBOXCL, nil
+	}
+	return MBOXO, nil
+}
diff --git a/detect_test.go b/detect_test.go
new file mode 100644
index 0000000..72634fe
--- /dev/null
+++ b/detect_test.go
@@ -0,0 +1,165 @@
+package mbox_test
+
+import (
+	"strings"
+	"testing"
+
+	"github.com/tvanriper/mbox"
+)
+
+func TestDetectRD(t *testing.T) {
+	mb := `From someone
+From: chuckles@funbunny.org
+To: hhefner@playboy.com
+Subject: Closed Captioning
+
+>From the engineers: do we really need closed captioning on this material?
+Please let me know, it'd save a lot of money if we could avoid it.
+
+From hhefner
+From: hhefner@playboy.com
+To: chuckles@funbunny.org
+Subject: RE: Closed Captioning
+
+Yes, silly as it sounds, we broadcast this material, and it must therefore have
+closed captioning.  The deaf will enjoy reading the material.
+`
+	mType, err := mbox.DetectType(strings.NewReader(mb))
+	if err != nil {
+		t.Error(err)
+	}
+	if mType != mbox.MBOXRD {
+		t.Errorf("expected %d but got %d", mbox.MBOXRD, mType)
+	}
+
+	// Ensure we also detect MBOXRD if the second mail has the character.
+	mb = `From someone
+From: chuckles@funbunny.org
+To: hhefner@playboy.com
+Subject: Closed Captioning
+
+Do we really need closed captioning on this material? Please let me know, it'd
+save a lot of money if we could avoid it.
+
+From hhefner
+From: hhefner@playboy.com
+To: chuckles@funbunny.org
+Subject: RE: Closed Captioning
+
+>From my lawyers: yes, silly as it sounds, we broadcast this material, and it
+must therefore have closed captioning.  The deaf will enjoy reading the
+material.
+`
+	mType, err = mbox.DetectType(strings.NewReader(mb))
+	if err != nil {
+		t.Error(err)
+	}
+	if mType != mbox.MBOXRD {
+		t.Errorf("expected %d but got %d", mbox.MBOXRD, mType)
+	}
+}
+
+func TestDetectCL(t *testing.T) {
+	mb := `From someone
+From: bubbles@bubbletown.com
+To: mrmxpdstk@lazytown.com
+Subject: To interpretation
+Content-Length: 33
+
+We should all try to enjoy life!
+From someone-else
+>From mug: weird header
+Content-Length: 130
+From: mrspam@corporate.corp.com
+To: mrmxpdstk@lazytown.com
+Subject: Bestest offer in the universe!!11!!
+
+You won't believe these prices!
+>From 1 cent to 11 cents, we carry the least expensive
+line of jets this side of the Gobi Desert!
+From nobody
+From: nobody@nowhere.man
+To: mrmxpdstk@lazytown.com
+Subject: Mysterious Jenkins
+Content-Length: 0
+
+`
+	mType, err := mbox.DetectType(strings.NewReader(mb))
+	if err != nil {
+		t.Error(err)
+	}
+	if mType != mbox.MBOXCL {
+		t.Errorf("expected %d but got %d", mbox.MBOXCL, mType)
+	}
+}
+
+func TestDetectCL2(t *testing.T) {
+	mb := `From someone
+From: bubbles@bubbletown.com
+To: mrmxpdstk@lazytown.com
+Subject: To interpretation
+Content-Length: 33
+
+We should all try to enjoy life!
+From someone-else
+>From mug: weird header
+Content-Length: 129
+From: mrspam@corporate.corp.com
+To: mrmxpdstk@lazytown.com
+Subject: Bestest offer in the universe!!11!!
+
+You won't believe these prices!
+From 1 cent to 11 cents, we carry the least expensive
+line of jets this side of the Gobi Desert!
+From nobody
+From: nobody@nowhere.man
+To: mrmxpdstk@lazytown.com
+Subject: Mysterious Jenkins
+Content-Length: 0
+
+`
+	mType, err := mbox.DetectType(strings.NewReader(mb))
+	if err != nil {
+		t.Error(err)
+	}
+	if mType != mbox.MBOXCL2 {
+		t.Errorf("expected %d but got %d", mbox.MBOXCL2, mType)
+	}
+}
+
+func TestDetectO(t *testing.T) {
+	mb := `From someone
+From: bubbles@bubbletown.com
+To: mrmxpdstk@lazytown.com
+Subject: To interpretation
+
+From all of us, to all of you, be happy!
+From someone-else
+From: mrspam@corporate.corp.com
+To: mrmxpdstk@lazytown.com
+Subject: Bestest offer in the universe!!11!!
+
+You won't believe these prices!
+From 1 cent to 11 cents, we carry the least expensive
+line of jets this side of the Gobi Desert!
+`
+	mType, err := mbox.DetectType(strings.NewReader(mb))
+	if err != nil {
+		t.Error(err)
+	}
+	if mType != mbox.MBOXO {
+		t.Errorf("expected %d but got %d", mbox.MBOXO, mType)
+	}
+}
+
+func TestDetectAltLinefeed(t *testing.T) {
+	mb := "From someone\r\nFrom: chuckles@funbunny.org\r\nTo: hhefner@playboy.com\r\nSubject: Closed Captioning\r\n\r\n>From the engineers: do we really need closed captioning on this material?\r\nPlease let me know, it'd save a lot of money if we could avoid it.\r\n\r\nFrom hhefner\r\nFrom: hhefner@playboy.com\r\nTo: chuckles@funbunny.org\r\nSubject: RE: Closed Captioning\r\n\r\nYes, silly as it sounds, we broadcast this material, and it must therefore have\r\nclosed captioning.  The deaf will enjoy reading the material.\r\n"
+	mType, err := mbox.DetectType(strings.NewReader(mb))
+	if err != nil {
+		t.Error(err)
+	}
+	if mType != mbox.MBOXRD {
+		t.Errorf("expected %d but got %d", mbox.MBOXRD, mType)
+	}
+
+}
diff --git a/example_mboxdetecttype_test.go b/example_mboxdetecttype_test.go
new file mode 100644
index 0000000..0f1a868
--- /dev/null
+++ b/example_mboxdetecttype_test.go
@@ -0,0 +1,50 @@
+package mbox_test
+
+import (
+	"bytes"
+	"fmt"
+
+	"github.com/tvanriper/mbox"
+)
+
+// Imagine this is a file on your filesystem instead of a variable in your code.
+const mboxrd2 string = `From bubbles@bubbletown.com Mon Jul 04 14:23:45 2022
+From: bubbles@bubbletown.com
+To: mrmxpdstk@lazytown.com
+Subject: To interpretation
+
+>From all of us, to all of you, be happy!
+From mrspam@corporate.corp.com Mon Jul 04 15:02:15 2022
+From: mrspam@corporate.corp.com
+To: mrmxpdstk@lazytown.com
+Subject: Bestest offer in the universe!!11!!
+
+You won't believe these prices!
+>From 1 cent to 11 cents, we carry the least expensive
+line of jets this side of the Gobi Desert!
+`
+
+func ExampleDetectType() {
+	// Imagine you used os.Open instead of bytes.NewBuffer here.
+	reader := bytes.NewReader([]byte(mboxrd2))
+	mbType, err := mbox.DetectType(reader)
+	if err != nil {
+		fmt.Printf("failed to detect mbox type: %s\n", err)
+		return
+	}
+	switch mbType {
+	case mbox.MBOXO:
+		fmt.Println("MBOXO")
+	case mbox.MBOXRD:
+		fmt.Println("MBOXRD")
+	case mbox.MBOXCL:
+		fmt.Println("MBOXCL")
+	case mbox.MBOXCL2:
+		fmt.Println("MBOXCL2")
+	default:
+		fmt.Println("Unknown")
+	}
+
+	// Output:
+	// MBOXRD
+}