The common word in these typically Arabic greetings is بعد : أمّا بعد ، وبعد ، تحيّة وبعد I think it means I write this letter after my best regards and sincere greetings or something like that . The last one isn’t correct you should write it : تحيّة واحترامًا ألف شكر

when you start with the letter you write تحية طيبة / or any other greetings then you write( اما بعد = after that )so you can start with the purpose from the letter (the object ) ^_^