diff --git a/imapserver/error.go b/imapserver/error.go
index 3f8e399..43b3068 100644
--- a/imapserver/error.go
+++ b/imapserver/error.go
@@ -1,6 +1,7 @@
 package imapserver
 
 import (
+	"errors"
 	"fmt"
 )
 
@@ -36,13 +37,14 @@ func xserverErrorf(format string, args ...any) {
 }
 
 type syntaxError struct {
-	line string // Optional line to write before BAD result. For untagged response. CRLF will be added.
-	code string // Optional result code (between []) to write in BAD result.
-	err  error  // BAD response message.
+	line   string // Optional line to write before BAD result. For untagged response. CRLF will be added.
+	code   string // Optional result code (between []) to write in BAD result.
+	errmsg string // BAD response message.
+	err    error  // Typically with same info as errmsg, but sometimes more.
 }
 
 func (e syntaxError) Error() string {
-	s := "bad syntax: " + e.err.Error()
+	s := "bad syntax: " + e.errmsg
 	if e.code != "" {
 		s += " [" + e.code + "]"
 	}
@@ -51,5 +53,7 @@ func (e syntaxError) Error() string {
 func (e syntaxError) Unwrap() error { return e.err }
 
 func xsyntaxErrorf(format string, args ...any) {
-	panic(syntaxError{"", "", fmt.Errorf(format, args...)})
+	errmsg := fmt.Sprintf(format, args...)
+	err := errors.New(errmsg)
+	panic(syntaxError{"", "", errmsg, err})
 }
diff --git a/imapserver/parse.go b/imapserver/parse.go
index 115d41b..d6236f6 100644
--- a/imapserver/parse.go
+++ b/imapserver/parse.go
@@ -1,6 +1,7 @@
 package imapserver
 
 import (
+	"errors"
 	"fmt"
 	"net/textproto"
 	"strconv"
@@ -74,11 +75,20 @@ func newParser(s string, conn *conn) *parser {
 }
 
 func (p *parser) xerrorf(format string, args ...any) {
-	var context string
+	var err error
+	errmsg := fmt.Sprintf(format, args...)
+	remaining := fmt.Sprintf("remaining %q", p.orig[p.o:])
 	if len(p.contexts) > 0 {
-		context = strings.Join(p.contexts, ",")
+		remaining += ", context " + strings.Join(p.contexts, ",")
 	}
-	panic(syntaxError{"", "", fmt.Errorf("%s (%sremaining data %q)", fmt.Sprintf(format, args...), context, p.orig[p.o:])})
+	remaining = " (" + remaining + ")"
+	if p.conn.account != nil {
+		errmsg += remaining
+		err = errors.New(errmsg)
+	} else {
+		err = errors.New(errmsg + remaining)
+	}
+	panic(syntaxError{"", "", errmsg, err})
 }
 
 func (p *parser) context(s string) func() {
@@ -724,7 +734,8 @@ func (p *parser) xliteralSize(maxSize int64, lit8 bool) (size int64, sync bool)
 	if maxSize > 0 && size > maxSize {
 		// ../rfc/7888:249
 		line := fmt.Sprintf("* BYE [ALERT] Max literal size %d is larger than allowed %d in this context", size, maxSize)
-		panic(syntaxError{line, "TOOBIG", fmt.Errorf("literal too big")})
+		err := errors.New("literal too big")
+		panic(syntaxError{line, "TOOBIG", err.Error(), err})
 	}
 
 	sync = !p.take("+")
diff --git a/imapserver/server.go b/imapserver/server.go
index ce6ac07..bdb0856 100644
--- a/imapserver/server.go
+++ b/imapserver/server.go
@@ -760,14 +760,21 @@ func (c *conn) command() {
 				c.writelinef("* BYE please try again speaking imap")
 				panic(errIO)
 			}
-			c.log.Debugx("imap command syntax error", err, logFields...)
+			c.log.Debugx("imap command syntax error", sxerr.err, logFields...)
 			c.log.Info("imap syntax error", mlog.Field("lastline", c.lastLine))
 			fatal := strings.HasSuffix(c.lastLine, "+}")
 			if fatal {
 				err := c.conn.SetWriteDeadline(time.Now().Add(5 * time.Second))
 				c.log.Check(err, "setting write deadline")
 			}
-			c.bwriteresultf("%s BAD %s unrecognized syntax/command: %v", tag, cmd, err)
+			if sxerr.line != "" {
+				c.bwritelinef("%s", sxerr.line)
+			}
+			code := ""
+			if sxerr.code != "" {
+				code = "[" + sxerr.code + "] "
+			}
+			c.bwriteresultf("%s BAD %s%s unrecognized syntax/command: %v", tag, code, cmd, sxerr.errmsg)
 			if fatal {
 				c.xflush()
 				panic(fmt.Errorf("aborting connection after syntax error for command with non-sync literal: %w", errProtocol))
@@ -1618,7 +1625,7 @@ func (c *conn) cmdAuthenticate(tag, cmd string, p *parser) {
 		c0 := xreadInitial()
 		ss, err := scram.NewServer(h, c0)
 		if err != nil {
-			xsyntaxErrorf("starting scram: %w", err)
+			xsyntaxErrorf("starting scram: %s", err)
 		}
 		c.log.Debug("scram auth", mlog.Field("authentication", ss.Authentication))
 		acc, _, err := store.OpenEmail(ss.Authentication)
diff --git a/smtpserver/error.go b/smtpserver/error.go
index 1bc43d7..8984190 100644
--- a/smtpserver/error.go
+++ b/smtpserver/error.go
@@ -8,23 +8,26 @@ import (
 
 func xcheckf(err error, format string, args ...any) {
 	if err != nil {
-		panic(smtpError{smtp.C451LocalErr, smtp.SeSys3Other0, fmt.Errorf("%s: %w", fmt.Sprintf(format, args...), err), true, false})
+		err := fmt.Errorf("%s: %w", fmt.Sprintf(format, args...), err)
+		panic(smtpError{smtp.C451LocalErr, smtp.SeSys3Other0, err.Error(), err, true, false})
 	}
 }
 
 type smtpError struct {
 	code       int
 	secode     string
-	err        error
+	errmsg     string // Sent in response.
+	err        error  // If set, used in logging. Typically has same information as errmsg.
 	printStack bool
 	userError  bool // If this is an error on the user side, which causes logging at a lower level.
 }
 
-func (e smtpError) Error() string { return e.err.Error() }
+func (e smtpError) Error() string { return e.errmsg }
 func (e smtpError) Unwrap() error { return e.err }
 
 func xsmtpErrorf(code int, secode string, userError bool, format string, args ...any) {
-	panic(smtpError{code, secode, fmt.Errorf(format, args...), false, userError})
+	err := fmt.Errorf(format, args...)
+	panic(smtpError{code, secode, err.Error(), err, false, userError})
 }
 
 func xsmtpServerErrorf(codes codes, format string, args ...any) {
diff --git a/smtpserver/parse.go b/smtpserver/parse.go
index 3c7bb99..7f614cf 100644
--- a/smtpserver/parse.go
+++ b/smtpserver/parse.go
@@ -1,6 +1,7 @@
 package smtpserver
 
 import (
+	"errors"
 	"fmt"
 	"net"
 	"strconv"
@@ -40,8 +41,19 @@ func newParser(s string, smtputf8 bool, conn *conn) *parser {
 }
 
 func (p *parser) xerrorf(format string, args ...any) {
+	// For submission, send the remaining unparsed line. Otherwise, only log it.
+	var err error
+	errmsg := "bad syntax: " + fmt.Sprintf(format, args...)
+	remaining := fmt.Sprintf(" (remaining %q)", p.orig[p.o:])
+	if p.conn.account != nil {
+		errmsg += remaining
+		err = errors.New(errmsg)
+	} else {
+		err = errors.New(errmsg + remaining)
+	}
+
 	// ../rfc/5321:2377
-	xsmtpUserErrorf(smtp.C501BadParamSyntax, smtp.SeProto5Syntax2, "%s (remaining: %q)", fmt.Sprintf(format, args...), p.orig[p.o:])
+	panic(smtpError{smtp.C501BadParamSyntax, smtp.SeProto5Syntax2, errmsg, err, false, true})
 }
 
 func (p *parser) xutf8localparterrorf() {
diff --git a/smtpserver/server.go b/smtpserver/server.go
index b1acc37..c69be90 100644
--- a/smtpserver/server.go
+++ b/smtpserver/server.go
@@ -669,7 +669,7 @@ func command(c *conn) {
 
 		var serr smtpError
 		if errors.As(err, &serr) {
-			c.writecodeline(serr.code, serr.secode, fmt.Sprintf("%s (%s)", serr.err, mox.ReceivedID(c.cid)), serr.err)
+			c.writecodeline(serr.code, serr.secode, fmt.Sprintf("%s (%s)", serr.errmsg, mox.ReceivedID(c.cid)), serr.err)
 			if serr.printStack {
 				debug.PrintStack()
 			}