Problem bei mail-Funktion mit PHP

Auf der Seite wird nochmal alles angezeigt und zur mail.php weitergeleitet, diese verschickt dann alles.
(Habe hier gekürzt.)
PHP:
                        <?php
 
 session_start();
 
 ?><form action="mail.php" method="post">
                          <table border="0" cellpadding="0" cellspacing="5" class="text-body">
                            <tr> 
                              <td width="20" align="left" valign="top">1.</td>
                              <td width="20">&nbsp;</td>
                              <td align="left" valign="top">Grundst&uuml;ck:</td>
                              <td width="20" align="left" valign="top">&nbsp;</td>
                              <td align="left" valign="top" class="text-bodyfett"><? echo $_SESSION["aa"]; ?></td>
                            </tr>
                            <tr> 
                              <td align="left" valign="top">2.</td>
                              <td>&nbsp;</td>
                              <td align="left" valign="top">Baurichtlinien:</td>
                              <td align="left" valign="top">&nbsp;</td>
                              <td align="left" valign="top" class="text-bodyfett"><? echo $_SESSION["ab"]; ?></td>
                            </tr>
                            <tr> 
                              <td align="left" valign="top">3.</td>
                              <td>&nbsp;</td>
                              <td align="left" valign="top">Grundst&uuml;cksbeschaffenheit:</td>
                              <td align="left" valign="top">&nbsp;</td>
                              <td align="left" valign="top" class="text-bodyfett"><? echo $_SESSION["ac"]; ?></td>
                            </tr>
                            <tr> 
                              <td align="left" valign="top">&nbsp;</td>
                              <td>&nbsp;</td>
                              <td align="left" valign="top">&nbsp;</td>
                              <td align="left" valign="top">&nbsp;</td>
                              <td align="left" valign="top" class="text-bodyfett">&nbsp;</td>
                            </tr>
                            <tr> 
                              <td align="left" valign="top">4.</td>
                              <td>&nbsp;</td>
                              <td align="left" valign="top">Haustyp:</td>
                              <td align="left" valign="top">&nbsp;</td>
                              <td align="left" valign="top" class="text-bodyfett"><? echo $_SESSION["ad"]; ?></td>
                            </tr>
                            <tr> 
                              <td align="left" valign="top">5.</td>
                              <td>&nbsp;</td>
                              <td align="left" valign="top">Anzahl der Geschosse:</td>
                              <td align="left" valign="top">&nbsp;</td>
                              <td align="left" valign="top" class="text-bodyfett"><? echo $_SESSION["ae"]; ?></td>
                            </tr>
                            <tr> 
                              <td align="left" valign="top">6.</td>
                              <td>&nbsp;</td>
                              <td align="left" valign="top">Fassade:</td>
                              <td align="left" valign="top">&nbsp;</td>
                              <td align="left" valign="top" class="text-bodyfett"><? echo $_SESSION["af"]; ?></td>
                            </tr>
                            <tr> 
                              <td align="left" valign="top">7.</td>
                              <td>&nbsp;</td>
                              <td align="left" valign="top">Bodenplatte oder Keller:</td>
                              <td align="left" valign="top">&nbsp;</td>
                              <td align="left" valign="top" class="text-bodyfett"><? echo $_SESSION["ag"]; ?></td>
                            </tr>
                            <tr> 
                              <td align="left" valign="top">8.</td>
                              <td>&nbsp;</td>
                              <td align="left" valign="top">Erker:</td>
                              <td align="left" valign="top">&nbsp;</td>
                              <td align="left" valign="top" class="text-bodyfett"><? echo $_SESSION["ah"]; ?></td>
                            </tr>
                            <tr> 
                              <td align="left" valign="top">9.</td>
                              <td>&nbsp;</td>
                              <td align="left" valign="top">Balkon:</td>
                              <td align="left" valign="top">&nbsp;</td>
                              <td align="left" valign="top" class="text-bodyfett"><? echo $_SESSION["ai"]; ?></td>
                            </tr>
                            <tr> 
                              <td align="left" valign="top">10.</td>
                              <td>&nbsp;</td>
                              <td align="left" valign="top">Garage oder Carport:</td>
                              <td align="left" valign="top">&nbsp;</td>
                              <td align="left" valign="top" class="text-bodyfett"><? echo $_SESSION["aj"]; ?></td>
                            </tr>
                            <tr> 
                              <td align="left" valign="top">11.</td>
                              <td>&nbsp;</td>
                              <td align="left" valign="top">Dacheindeckung:</td>
                              <td align="left" valign="top">&nbsp;</td>
                              <td align="left" valign="top" class="text-bodyfett"><? echo $_SESSION["ak"]; ?></td>
                            </tr>
                            <tr> 
                              <td align="left" valign="top">12.</td>
                              <td>&nbsp;</td>
                              <td align="left" valign="top">Fenster:</td>
                              <td align="left" valign="top">&nbsp;</td>
                              <td align="left" valign="top" class="text-bodyfett"><? echo $_SESSION["al"]; ?></td>
                            </tr>
                          </table> 
                        <br>
                        <table width="400" border="0" cellspacing="0" cellpadding="0">
                          <tr> 
                            <td><a href="f3.php" target="_self" class="text-link">Zur&uuml;ck</a></td>
                            <td><input name="submit" type="submit" value="kostenlose Analyse abschicken"/></td>
                          </tr>
                        </table>
                        <br>
                        <br></form>
 
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