me.com mail aliases disappeared

Hi there,

after upgrading my iMac to El Capitan my mail aliases disappeared from the account-settings in Apple Mail. I don't use my primary mail address for the iCloud-account, but an alias. Now this has disappeared from the account settings. I do receive mails addressed to the alias, but I can't send from it.

On my MacBook Air everything is o.k., after upgrading to El Capitan all aliases are still there.

Has anyone a fix for my problem? That would be great.

Thx in advance, cheers

Thomas