tried to send email by using forward button now the email is flashing on and off

Please disconnect from the Internet temporarily, if necessary, by turning off Wi-Fi and/or unplugging the Ethernet cable, whichever is applicable.

From the Mail menu bar, select

Window ▹ Connection Doctor

Look for the outgoing mail (SMTP) account in the window that opens. Double-click it. Another window will open, showing a list of all the outgoing mail accounts. The affected one will be selected. Make a note of the settings. Click the minus-sign button to delete it, then click OK.

Reconnect to the Internet and add the account back with the same settings.