It looks like there is a restriction in the Thunderbird code that limits the last part of the address to maximum 6 characters, so at the moment it is not possible to use addresses like: .academy .accountant .airforce .apartments etc... I've filed a bug for this issue, so I hope it will be fixed in a future version of Thunderbird: [https://bugzil.la/1379976 Bug 1379976 - New Mail Account wizard doesn't allow setup of .academy mail address] Maybe you can try manual setup and see if that works for now?