Tu peux essayer https://www.mail-tester.com pour voir les erreurs qu'il te remonte. Ainsi que https://mxtoolbox.com ou http://www.pagasa.net/test-smtp. Bref, y a de quoi identifier certains problèmes ;)

Merci, J'ai remarqué que les erreurs :

Mail Delivery System
<"'heaven.wang@symphonylimited.com.'"@se-in3.mx-wecloud.net>: host
se-in3.mx-wecloud.net[89.221.255.91] said: 554 5.7.1
<'heaven.wang@symphonylimited.com.'@se-in3.mx-wecloud.net>: Relay access
denied (in reply to RCPT TO command)

ou

````jacky@hzhongtai.com: host mx.c1.icoremail.net[61.145.121.60] said: 550
User
not found; (in reply to MAIL FROM command)

yoyo@hzhongtai.com: host mx.c1.icoremail.net[61.145.121.60] said: 550 User
not found; (in reply to MAIL FROM command)````

ou

````jacky@hzhongtai.com: host mx.c1.icoremail.net[61.145.121.60] said: 421
Read
data from client error (in reply to end of DATA command)

yoyo@hzhongtai.com: host mx.c1.icoremail.net[61.145.121.60] said: 421 Read
data from client error (in reply to end of DATA command)````

Viennent seulement (pas certain à 100%) quand j'utilise Outlook, jamais avec Thunderbird ou le web mail bizarre non?