The IMAP method allows user names with or without a domain part. For
user names without a domain part, it still tries to access the (then
non-existing) domain, resulting in "Error: Undefined array key 1 at
.../nextcloud/apps/user_external/lib/IMAP.php#85" log messages.
Signed-off-by: Bjoern Kahl <mls@bjoern-kahl.de>
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