DKIM no roundcube

Quando conectando via POP3 / IMAP externamente (ou seja, Thunderbird de não localhost) o relatório da porta 25 eu recebo é

==========================================================
Summary of Results
==========================================================
SPF check:          pass
DomainKeys check:   pass
DKIM check:         pass
Sender-ID check:    pass
SpamAssassin check: ham

==========================================================
Details:
==========================================================

HELO hostname:  server.co.uk
Source IP:      ipaddress
mail-from:      [email protected]

----------------------------------------------------------
SPF check details:
----------------------------------------------------------
Result:         pass 
ID(s) verified: [email protected]
DNS record(s):
    server.co.uk. SPF (no records)
    server.co.uk. 86000 IN TXT "v=spf1 a mx  ~all"
    server.co.uk. 86000 IN A ip.address

----------------------------------------------------------
DomainKeys check details:
----------------------------------------------------------
Result:         pass 
ID(s) verified: [email protected]
DNS record(s):
    private._domainkey.server.co.uk. 86000 IN TXT "k=rsa; p=KEYHERE"

----------------------------------------------------------
DKIM check details:
----------------------------------------------------------
Result:         pass (signature verifies; identity doesn't match any headers)
ID(s) verified: header.d=server.co.uk
Canonicalized Headers:
    Message-ID:'20'<[email protected]>'0D''0A'
    Date:'20'Sun,'20'26'20'Jan'20'2014'20'12:04:37'20'+0000'0D''0A'
    From:'20'Name'20'Name'20'<[email protected]>'0D''0A'
    MIME-Version:'20'1.0'0D''0A'
    To:'20'[email protected]'0D''0A'
    Subject:'20'Authentication'20'Test'0D''0A'
    Content-Type:'20'text/plain;'20'charset=ISO-8859-1;'20'format=flowed'0D''0A'
    Content-Transfer-Encoding:'20'7bit'0D''0A'
    DKIM-Signature:'20'v=1;'20'a=rsa-sha1;'20'c=simple;'20'd=server.co.uk;'20'h='0D''0A'
    '09'message-id:date:from:mime-version:to:subject:content-type'0D''0A'
    '09':content-transfer-encoding;'20's=dkim1;'20'bh=O3blcDPl7o3m4jKKiKXmgtMo'0D''0A'
    '09'LLs=;'20'b=

Canonicalized Body:
    Thunderbird'0D''0A'


DNS record(s):
    dkim1._domainkey.server.co.uk. 14400 IN TXT "k=rsa; p=KEY"

Public key used for verification: dkim1._domainkey.server.co.uk (384 bits)

NOTE: DKIM checking has been performed based on the latest DKIM specs
(RFC 4871 or draft-ietf-dkim-base-10) and verification may fail for
older versions.  If you are using Port25's PowerMTA, you need to use
version 3.2r11 or later to get a compatible version of DKIM.

----------------------------------------------------------
Sender-ID check details:
----------------------------------------------------------
Result:         pass 
ID(s) verified: [email protected]
DNS record(s):
    server.co.uk. SPF (no records)
    server.co.uk. 86000 IN TXT "v=spf1 a mx  ~all"
    server.co.uk. 86000 IN A ip.address

----------------------------------------------------------
SpamAssassin check details:
----------------------------------------------------------
SpamAssassin v3.3.1 (2010-03-16)

Result:         ham  (-2.0 points, 5.0 required)

 pts rule name              description
---- ---------------------- --------------------------------------------------
-1.9 BAYES_00               BODY: Bayes spam probability is 0 to 1%
                            [score: 0.0000]
-0.1 DKIM_VALID_AU          Message has a valid DKIM or DK signature from author's
                            domain
 0.1 DKIM_SIGNED            Message has a DKIM or DK signature, not necessarily valid
-0.1 DKIM_VALID             Message has at least one valid DKIM or DK signature

E tudo parece bem.

Ao usar o Roundcube, recebo o seguinte

==========================================================
Summary of Results
==========================================================
SPF check:          pass
DomainKeys check:   neutral
DKIM check:         fail
Sender-ID check:    pass
SpamAssassin check: ham

==========================================================
Details:
==========================================================

HELO hostname:  server.co.uk
Source IP:      ip.address
mail-from:      [email protected]

----------------------------------------------------------
SPF check details:
----------------------------------------------------------
Result:         pass 
ID(s) verified: [email protected]
DNS record(s):
    server.co.uk. SPF (no records)
    server.co.uk. 86000 IN TXT "v=spf1 a mx  ~all"
    server.co.uk. 86000 IN A ip.address

----------------------------------------------------------
DomainKeys check details:
----------------------------------------------------------
Result:         neutral (message not signed)
ID(s) verified: [email protected]
DNS record(s):

----------------------------------------------------------
DKIM check details:
----------------------------------------------------------
Result:         fail (signature doesn't verify)
ID(s) verified: 
Canonicalized Headers:
    To:'20'<[email protected]>'0D''0A'
    Subject:'20'Check'0D''0A'
    MIME-Version:'20'1.0'0D''0A'
    Content-Type:'20'text/plain;'20'charset=UTF-8;'0D''0A'
    '20'format=flowed'0D''0A'
    Content-Transfer-Encoding:'20'7bit'0D''0A'
    Date:'20'Sun,'20'26'20'Jan'20'2014'20'12:05:26'20'+0000'0D''0A'
    From:'20'USER'20'USER'20'<[email protected]>'0D''0A'
    Message-ID:'20'<[email protected]>'0D''0A'
    DKIM-Signature:'20'v=1;'20'a=rsa-sha1;'20'c=simple;'20'd=server.co.uk;'20'h=to'0D''0A'
    '09':subject:mime-version:content-type:content-transfer-encoding'0D''0A'
    '09':date:from:message-id;'20's=dkim1;'20'bh=4kh3s4BsRzvISfGlbXqWNi6G4cg=;'20'b='0D''0A'
    '09'

Canonicalized Body:
    Roundcube'0D''0A'


DNS record(s):
    dkim1._domainkey.server.co.uk. 14400 IN TXT "k=rsa; p=KEY"

Public key used for verification: dkim1._domainkey.server.co.uk (384 bits)

NOTE: DKIM checking has been performed based on the latest DKIM specs
(RFC 4871 or draft-ietf-dkim-base-10) and verification may fail for
older versions.  If you are using Port25's PowerMTA, you need to use
version 3.2r11 or later to get a compatible version of DKIM.

----------------------------------------------------------
Sender-ID check details:
----------------------------------------------------------
Result:         pass 
ID(s) verified: [email protected]
DNS record(s):
    server.co.uk. SPF (no records)
    server.co.uk. 86000 IN TXT "v=spf1 a mx  ~all"
    server.co.uk. 86000 IN A ip.address

----------------------------------------------------------
SpamAssassin check details:
----------------------------------------------------------
SpamAssassin v3.3.1 (2010-03-16)

Result:         ham  (-1.9 points, 5.0 required)

 pts rule name              description
---- ---------------------- --------------------------------------------------
-1.9 BAYES_00               BODY: Bayes spam probability is 0 to 1%
                            [score: 0.0000]
 0.1 DKIM_SIGNED            Message has a DKIM or DK signature, not necessarily valid
-0.1 DKIM_VALID             Message has at least one valid DKIM or DK signature

Pelo que eu posso dizer, o Roundcube se conecta ao IMAP para ter 100% de certeza de qual é a diferença para que ele seja correto externamente, e não do localhost

O que eu poderia fazer para fazer o roundcube enviar mensagens corretamente?

Editar

// use this host for sending mails. 
// to use SSL connection, set ssl://smtp.host.com 
// if left blank, the PHP mail() function is used 
$rcmail_config['smtp_server'] = 'mail.server.co.uk'; 

// SMTP port (default is 25; 465 for SSL) 
$rcmail_config['smtp_port'] = 25; 

// SMTP username (if required) if you use %u as the username RoundCube 
// will use the current username for login 
$rcmail_config['smtp_user'] = '%u'; 

// SMTP password (if required) if you use %p as the password RoundCube 
// will use the current user's password for login 
$rcmail_config['smtp_pass'] = '%p'; 

// SMTP AUTH type (DIGEST-MD5, CRAM-MD5, LOGIN, PLAIN or empty to use 
// best server supported one) 
$rcmail_config['smtp_auth_type'] = ''; 

// SMTP HELO host  
// Hostname to give to the remote server for SMTP 'HELO' or 'EHLO' messages  
// Leave this blank and you will get the server variable 'server_name' or  
// localhost if that isn't defined.  
$rcmail_config['smtp_helo_host'] = ''; 

// Log sent messages 
$rcmail_config['smtp_log'] = TRUE; 

Itens de configuração do roundcube relacionados ao SMTP