Hello,
I can´t receive E-Mails from different accounts. Some work, but three won´t. I did not change any settings.
The message: [IMAP] MailClient.Accounts.ConnectionException: Der Server unterstützt keine sichere Verbindung, dadurch kann keine sichere Verbindung hergestellt werden.
The fact that you “did not change any settings” begs the question as to whether you have contacted/visited the email hosts to make sure they have not changed anything with connection settings, etc. It is not unheard of for email hosts to not notify you of changes.
Posting account settings of those that are problematic, along with the host names of those email hosts will give us some information to start from.