Interestingly looking at my logs I can see that it misses out the email address its sending from:

 

[2016.06.01] 07:48:12 [63352] Delivery started for  at 7:48:12 AM
[2016.06.01] 07:48:15 [63352] Error checking SPF Record: Spf check failed due to null sender's domain
[2016.06.01] 07:48:16 [63352] Spam check results: [BARRACUDA - BRBL: passed], [CBL - ABUSE SEAT - DO NOT USE FOR OUTGOING!: passed], [HOSTKARMA - BLACKLIST: passed], [MAILSPIKE Z: passed], [PROTECTED SKY - RED: passed], [RFC2 REALTIME LIST: passed], [SORBS 02 - HTTP: passed], [SORBS 03 - SOCKS: passed], [SORBS 05 - SMTP: passed], [SORBS 08 - BLOCK: passed], [SORBS 09 - ZOMBIE: passed], [SORBS 11 - BAD CONFIG: passed], [SORBS 12 - NOMAIL: passed], [SORBS 13 - NO SERVER: passed], [SPAMHAUS - PBL 1: passed], [SPAMHAUS - PBL 2: passed], [SPAMHAUS - SBL 1: passed], [SPAMHAUS - SBL 2: passed], [SPAMHAUS - XBL 1: passed], [SPAMHAUS - XBL 2: passed], [SPAMHAUS - XBL 3: passed], [SPAMHAUS - XBL 4: passed], [SPAMHAUS ZEN: passed], [SPAMRATS: passed], [SURRIEL: passed], [VIRUS RBL - MSRBL: passed], [_REVERSEDNSLOOKUP: passed], [_BAYESIANFILTERING: passed], [_MESSAGESNIFFER: 0,code:0], [_DK: None], [_DKIM: Pass], [NOABUSE: passed], [NOPOSTMASTER: passed], [PROTECTED SKY - YELLOW: passed], [SEM-URIBL: passed], [SEM-URIRED: passed], [SORBS 04 - MISC: passed], [SORBS 06 - RECENT: passed], [SORBS 07 - WEB: passed], [SORBS 10 - DYNAMIC IP: passed], [UCEPROTECT LEVEL 1: passed], [UCEPROTECT LEVEL 2: passed], [UCEPROTECT LEVEL 3: passed]
[2016.06.01] 07:48:18 [63352] Starting local delivery to paul.smith@nwev.co.uk
[2016.06.01] 07:48:18 [63352] Delivery for  to paul.smith@nwev.co.uk has completed (Forwarded Delivered) Filter: None
[2016.06.01] 07:48:18 [63352] End delivery to paul.smith@nwev.co.uk
[2016.06.01] 07:48:18 [63352] Sending remote mail for 
[2016.06.01] 07:48:18 [63352] Initiating connection to 65.55.37.88
[2016.06.01] 07:48:18 [63352] Connecting to 65.55.37.88:25 (Id: 1)
[2016.06.01] 07:48:18 [63352] Binding to local IP 217.160.95.61:0 (Id: 1)
[2016.06.01] 07:48:18 [63352] Connection to 65.55.37.88:25 from 217.160.95.61:58152 succeeded (Id: 1)
[2016.06.01] 07:48:18 [63352] RSP: 220 COL004-MC2F47.hotmail.com Sending unsolicited commercial or bulk e-mail to Microsoft's computer network is prohibited. Other restrictions are found at http://privacy.microsoft.com/en-us/anti-spam.mspx. Tue, 31 May 2016 23:48:29 -0700 
[2016.06.01] 07:48:18 [63352] CMD: EHLO mail.xlvets.co.uk
[2016.06.01] 07:48:19 [63352] RSP: 250-COL004-MC2F47.hotmail.com (3.21.0.244) Hello [217.160.95.61]
[2016.06.01] 07:48:19 [63352] RSP: 250-SIZE 36909875
[2016.06.01] 07:48:19 [63352] RSP: 250-PIPELINING
[2016.06.01] 07:48:19 [63352] RSP: 250-8bitmime
[2016.06.01] 07:48:19 [63352] RSP: 250-BINARYMIME
[2016.06.01] 07:48:19 [63352] RSP: 250-CHUNKING
[2016.06.01] 07:48:19 [63352] RSP: 250-STARTTLS
[2016.06.01] 07:48:19 [63352] RSP: 250-AUTH LOGIN
[2016.06.01] 07:48:19 [63352] RSP: 250-AUTH=LOGIN
[2016.06.01] 07:48:19 [63352] RSP: 250 OK
[2016.06.01] 07:48:19 [63352] CMD: STARTTLS
[2016.06.01] 07:48:19 [63352] RSP: 220 SMTP server ready
[2016.06.01] 07:48:19 [63352] CMD: EHLO mail.xlvets.co.uk
[2016.06.01] 07:48:19 [63352] RSP: 250-COL004-MC2F47.hotmail.com (3.21.0.244) Hello [217.160.95.61]
[2016.06.01] 07:48:19 [63352] RSP: 250-SIZE 36909875
[2016.06.01] 07:48:19 [63352] RSP: 250-PIPELINING
[2016.06.01] 07:48:19 [63352] RSP: 250-8bitmime
[2016.06.01] 07:48:19 [63352] RSP: 250-BINARYMIME
[2016.06.01] 07:48:19 [63352] RSP: 250-CHUNKING
[2016.06.01] 07:48:19 [63352] RSP: 250-AUTH LOGIN
[2016.06.01] 07:48:19 [63352] RSP: 250-AUTH=LOGIN
[2016.06.01] 07:48:19 [63352] RSP: 250 OK
[2016.06.01] 07:48:19 [63352] CMD: MAIL FROM:<> SIZE=2630
[2016.06.01] 07:48:20 [63352] RSP: 250 <>....Sender OK
[2016.06.01] 07:48:20 [63352] CMD: RCPT TO:<p.t.s.smith@hotmail.co.uk>
[2016.06.01] 07:48:20 [63352] RSP: 250 p.t.s.smith@hotmail.co.uk 
[2016.06.01] 07:48:20 [63352] CMD: DATA
[2016.06.01] 07:48:20 [63352] RSP: 354 Start mail input; end with <CRLF>.<CRLF>
[2016.06.01] 07:48:20 [63352] RSP: 250  <fedb52e6145c4ebb9ca6fdda421da9eb@GBMACEUHUBD0308.rbsres01.net> Queued mail for delivery
[2016.06.01] 07:48:20 [63352] CMD: QUIT
[2016.06.01] 07:48:21 [63352] RSP: 221 COL004-MC2F47.hotmail.com Service closing transmission channel
[2016.06.01] 07:48:21 [63352] Delivery for  to p.t.s.smith@hotmail.co.uk has completed (Delivered)
[2016.06.01] 07:48:21 [63352] Delivery finished for  at 7:48:21 AM    [id:47806463352]

 

This domain does not have an SPF file nwev.co.uk. All domains should have and SPF and DMarc, this will stop spoofing of your domains.

 

You can test SPF here http://www.kitterman.com/spf/validate.html

 

You can turn off SPF checker in SM at Antispam Administration/Options/

 

You can learn about SPF record here. http://www.openspf.org/SPF_Record_Syntax

 

Good site, it will make suggestions on how to shorten the spf  https://dmarcian.com/spf-survey/nwev.co.uk