更新：由於還沒時間完成新手訓練，所以沒辦法回覆大家的回答..

謝謝大家的回覆！看了大家的回答，看來真的很多需要debug的，感覺是我太急所以很多步驟都有點錯亂了，我已決定打掉重練，先照echochio大說的去裝裝看iredmail（畢竟也是沒裝過..）
很感謝大家的回答！！！

各位前輩大家好！
最近在Linux Debian10上架設了mail server
一步一步照著教學做了，結果收、發信還是都不行..
目前Postfix有正常運作
（雖然顯示exited，但爬文後好像這是Debian的bug?似乎不影響?)
Dovecot跟MySQL也有正常運作
查了maillog顯示下面這樣

Feb  7 04:35:44 localhost postfix/cleanup[24415]: fatal: trace: remove 3470F1ECCF log: Permission denied
Feb  7 04:35:44 localhost postfix/cleanup[24416]: fatal: trace: remove 3795E1ECD2 log: Permission denied
Feb  7 04:35:45 localhost postfix/master[22985]: warning: process /usr/lib/postfix/sbin/cleanup pid 24415 exit status 1
Feb  7 04:35:45 localhost postfix/master[22985]: warning: /usr/lib/postfix/sbin/cleanup: bad command startup -- throttling
Feb  7 04:35:45 localhost postfix/master[22985]: warning: process /usr/lib/postfix/sbin/cleanup pid 24416 exit status 1
Feb  7 04:35:45 localhost postfix/smtpd[24402]: fatal: unable to connect to the public cleanup service
Feb  7 04:35:46 localhost postfix/master[22985]: warning: process /usr/lib/postfix/sbin/smtpd pid 24402 exit status 1
Feb  7 04:35:46 localhost postfix/master[22985]: warning: /usr/lib/postfix/sbin/smtpd: bad command startup -- throttling
Feb  7 04:35:46 localhost postfix/cleanup[24417]: fatal: trace: remove 395B41ECCF log: Permission denied
Feb  7 04:35:47 localhost postfix/master[22985]: warning: process /usr/lib/postfix/sbin/cleanup pid 24417 exit status 1
Feb  7 04:35:54 localhost postfix/master[22985]: warning: master_wakeup_timer_event: service pickup(public/pickup): Resource temporarily unavailable
Feb  7 04:36:02 localhost postfix/qmgr[24426]: fatal: scan_dir_push: open directory deferred/7: Permission denied
Feb  7 04:36:03 localhost postfix/master[22985]: warning: process /usr/lib/postfix/sbin/qmgr pid 24426 exit status 1
Feb  7 04:36:03 localhost postfix/master[22985]: warning: /usr/lib/postfix/sbin/qmgr: bad command startup -- throttling
Feb  7 04:36:46 localhost postfix/cleanup[24434]: fatal: defer: remove 484BE1ECCF log: Permission denied
Feb  7 04:36:47 localhost postfix/master[22985]: warning: process /usr/lib/postfix/sbin/cleanup pid 24434 exit status 1
Feb  7 04:36:47 localhost postfix/master[22985]: warning: /usr/lib/postfix/sbin/cleanup: bad command startup -- throttling
Feb  7 04:36:48 localhost postfix/cleanup[24435]: fatal: defer: remove 4CE921ECCF log: Permission denied
Feb  7 04:36:49 localhost postfix/master[22985]: warning: process /usr/lib/postfix/sbin/cleanup pid 24435 exit status 1
Feb  7 04:36:54 localhost postfix/master[22985]: warning: master_wakeup_timer_event: service pickup(public/pickup): Resource temporarily unavailable

又看了一下postqueue -p，全都沒寄出去..
再麻煩各位前輩們解惑了！
剛接觸這方面，有什麼缺少的資料再麻煩告知！非常感謝～～～