I am still facing the above problem. The setting is not 100 % clear to me. I do know that t-online requires an additional password for using other than Apple mail etc (thunderbird). So initially I used this extra password when trying to add my t-onlne e-mail, but this failed, likewise it did with the “first” t-online password. I did check the settings imap etc with my t-online settings which I manually changed, to no avail.
Today I sent an e-mail directly vom T-online webpage to eM client which did arrive in the inbox of my gmx- e-mail account registered in eM client. Any ideas …?