Distribution Group receives NDR for external senders and some addresses

I have a mail universal distribution group alias@domain.com on a Exchange 2013 server. Later, otheralias@domain.com was added as address and configured as primary SMTP address. External senders are, however, permitted; an e-mail address policy is not enabled nor is moderation.

RunspaceId                             : d2c70b0f-ed57-4b30-bc55-d2a33b1d8545
GroupType                              : Universal
SamAccountName                         : ***
BypassNestedModerationEnabled          : False
ManagedBy                              : ***
MemberJoinRestriction                  : Closed
MemberDepartRestriction                : Closed
ExpansionServer                        :
ReportToManagerEnabled                 : False
ReportToOriginatorEnabled              : True
SendOofMessageToOriginatorEnabled      : False
AcceptMessagesOnlyFrom                 : {}
AcceptMessagesOnlyFromDLMembers        : {}
AcceptMessagesOnlyFromSendersOrMembers : {}
AddressListMembership                  : {\Groups(VLV), \All Groups(VLV), \All Recipients(VLV), \All Distribution
                                         Lists, \Default Global Address List}
Alias                                  : ***
ArbitrationMailbox                     : ***
BypassModerationFromSendersOrMembers   : {}
OrganizationalUnit                     : ***
CustomAttribute1                       :
CustomAttribute10                      :
CustomAttribute11                      :
CustomAttribute12                      :
CustomAttribute13                      :
CustomAttribute14                      :
CustomAttribute15                      :
CustomAttribute2                       :
CustomAttribute3                       :
CustomAttribute4                       :
CustomAttribute5                       :
CustomAttribute6                       :
CustomAttribute7                       :
CustomAttribute8                       :
CustomAttribute9                       :
ExtensionCustomAttribute1              : {}
ExtensionCustomAttribute2              : {}
ExtensionCustomAttribute3              : {}
ExtensionCustomAttribute4              : {}
ExtensionCustomAttribute5              : {}
DisplayName                            : ***
EmailAddresses                         : ***
GrantSendOnBehalfTo                    : ***
ExternalDirectoryObjectId              :
HiddenFromAddressListsEnabled          : False
LastExchangeChangedTime                :
LegacyExchangeDN                       : ***
MaxSendSize                            : Unlimited
MaxReceiveSize                         : Unlimited
ModeratedBy                            : {}
ModerationEnabled                      : False
PoliciesIncluded                       : {}
PoliciesExcluded                       : {{26491cfc-9e50-4857-861b-0cb8df22b5d7}}
EmailAddressPolicyEnabled              : False
PrimarySmtpAddress                     : ***
RecipientType                          : MailUniversalDistributionGroup
RecipientTypeDetails                   : MailUniversalDistributionGroup
RejectMessagesFrom                     : {}
RejectMessagesFromDLMembers            : {}
RejectMessagesFromSendersOrMembers     : {}
RequireSenderAuthenticationEnabled     : False
SimpleDisplayName                      :
SendModerationNotifications            : Always
UMDtmfMap                              : {emailAddress:732877161, lastNameFirstName:732877161,
                                         firstNameLastName:732877161}
WindowsEmailAddress                    : ***
MailTip                                :
MailTipTranslations                    : {}
Identity                               : ***
IsValid                                : True
ExchangeVersion                        : 0.10 (14.0.100.0)
Name                                   : ***
DistinguishedName                      : ***
Guid                                   : ca7230e2-dbb1-497d-8e1d-443893397314
ObjectCategory                         : ***
ObjectClass                            : {top, group}
WhenChanged                            : 28.10.2015 15:39:17
WhenCreated                            : 09.06.2015 16:54:08
WhenChangedUTC                         : 28.10.2015 14:39:17
WhenCreatedUTC                         : 09.06.2015 14:54:08
OrganizationId                         :
Id                                     : ***
OriginatingServer                      : ***
ObjectState                            : Changed

The problem occurring now is that I can send mails to alias@domain.com from within and outside the organisation. I can also send mails to otheralias@domain.com from within the organisation. However, I cannot sendto otheralias@domain.com from outside the organisation. The server creates an NDR with

Diagnostic-Code: smtp;550 5.1.1 RESOLVER.ADR.RecipNotFound; not found

How can I fix that?

Thanks in advance,
Christoph