Você tem isso no seu imapd.conf?
loginrealms: fqdn.example.com
(a página do manual imapd.conf diz isso):
loginrealms: <empty string>
The list of remote realms whose users may authenticate using cross-realm authentication identifiers. Seperate each realm name by a space. (A cross-realm iden‐
tity is considered any identity returned by SASL with an "@" in it.) Note that to support multiple virtual domains on the same interface/IP, you need to list them
all as loginreals. If you don’t list them here, your users probably won’t be able to log in.