diff --git a/email.go b/email.go
index 69186ac..b29f487 100644
--- a/email.go
+++ b/email.go
@@ -12,6 +12,7 @@ import (
 	"net/mail"
 	"net/textproto"
 	"path/filepath"
+	"strconv"
 	"time"
 )
 
@@ -970,8 +971,15 @@ func send(from string, to []string, msg string, client *SMTPClient) error {
 }
 
 func sendMailProcess(from string, to []string, msg string, c *smtpClient) error {
+
+	cmdArgs := make(map[string]string)
+
+	if _, ok := c.ext["SIZE"]; ok {
+		cmdArgs["SIZE"] = strconv.Itoa(len(msg))
+	}
+
 	// Set the sender
-	if err := c.mail(from); err != nil {
+	if err := c.mail(from, cmdArgs); err != nil {
 		return err
 	}
 
diff --git a/smtp.go b/smtp.go
index 30ba1cf..e9a38bb 100644
--- a/smtp.go
+++ b/smtp.go
@@ -8,6 +8,7 @@
 //	SMTPUTF8  RFC 6531
 //	AUTH      RFC 2554
 //	STARTTLS  RFC 3207
+//  SIZE      RFC 1870
 // Additional extensions may be handled by clients using smtp.go in golang source code or pull request Go Simple Mail
 
 // smtp.go file is a modification of smtp golang package what is frozen and is not accepting new features.
@@ -206,7 +207,14 @@ func (c *smtpClient) authenticate(a auth) error {
 // If the server supports the SMTPUTF8 extension, Mail adds the
 // SMTPUTF8 parameter.
 // This initiates a mail transaction and is followed by one or more Rcpt calls.
-func (c *smtpClient) mail(from string) error {
+func (c *smtpClient) mail(from string, extArgs ...map[string]string) error {
+	var args []interface{}
+	var extMap map[string]string
+
+	if len(extArgs) > 0 {
+		extMap = extArgs[0]
+	}
+
 	if err := validateLine(from); err != nil {
 		return err
 	}
@@ -221,8 +229,15 @@ func (c *smtpClient) mail(from string) error {
 		if _, ok := c.ext["SMTPUTF8"]; ok {
 			cmdStr += " SMTPUTF8"
 		}
+		if _, ok := c.ext["SIZE"]; ok {
+			if extMap["SIZE"] != "" {
+				cmdStr += " SIZE=%s"
+				args = append(args, extMap["SIZE"])
+			}
+		}
 	}
-	_, _, err := c.cmd(250, cmdStr, from)
+	args = append([]interface{}{from}, args...)
+	_, _, err := c.cmd(250, cmdStr, args...)
 	return err
 }
 
diff --git a/smtp_test.go b/smtp_test.go
index ac331aa..54feead 100644
--- a/smtp_test.go
+++ b/smtp_test.go
@@ -397,6 +397,52 @@ QUIT
 		}
 	})
 
+	t.Run("ehlo size", func(t *testing.T) {
+		const (
+			basicServer = `250-mx.google.com at your service
+250-SIZE 35651584
+250 user@gmail.com OK; can accommodate 500000 byte message
+250 Sender OK
+221 Goodbye
+`
+
+			basicClient = `EHLO localhost
+MAIL FROM:<user@gmail.com> SIZE=50000
+QUIT
+`
+		)
+
+		c, bcmdbuf, cmdbuf := faker(basicServer)
+		cmdArgs := make(map[string]string)
+
+		if err := c.hi("localhost"); err != nil {
+			t.Fatalf("EHLO failed: %s", err)
+		}
+		if ok, _ := c.extension("SIZE"); !ok {
+			t.Fatalf("Should support SIZE")
+		}
+		if ok, _ := c.extension("SMTPUTF8"); ok {
+			t.Fatalf("Shouldn't support SMTPUTF8")
+		}
+
+		if ok, _ := c.extension("SIZE"); ok {
+			cmdArgs["SIZE"] = "50000"
+		}
+		if err := c.mail("user@gmail.com", cmdArgs); err != nil {
+			t.Fatalf("MAIL FROM failed: %s", err)
+		}
+		if err := c.quit(); err != nil {
+			t.Fatalf("QUIT failed: %s", err)
+		}
+
+		bcmdbuf.Flush()
+		actualcmds := cmdbuf.String()
+		client := strings.Join(strings.Split(basicClient, "\n"), "\r\n")
+		if client != actualcmds {
+			t.Fatalf("Got:\n%s\nExpected:\n%s", actualcmds, client)
+		}
+	})
+
 	t.Run("ehlo 8bitmime", func(t *testing.T) {
 		const (
 			basicServer = `250-mx.google.com at your service