Looking at the logs:
Source email as service@alt.zensupport.co.uk
May 30 16:25:03 debian postfix/smtpd[14518]: connect from smarthost01.mail.zen.net.uk[212.23.1.1]
May 30 16:25:04 debian postfix/smtpd[14518]: F2A8311667: client=smarthost01.mail.zen.net.uk[212.23.1.1]
May 30 16:25:04 debian postfix/cleanup[14521]: F2A8311667: message-id=testmail22177@zensupport.co.uk
May 30 16:25:04 debian postfix/qmgr[2636]: F2A8311667: from=service@alt.zensupport.co.uk, size=892, nrcpt=1 (queue active)
May 30 16:25:04 debian postfix/smtpd[14518]: disconnect from smarthost01.mail.zen.net.uk[212.23.1.1]
May 30 16:25:04 debian spamd[11425]: spamd: connection from localhost [127.0.0.1] at port 47805
May 30 16:25:04 debian spamd[11425]: spamd: setuid to ads.norab succeeded
May 30 16:25:04 debian spamd[11425]: spamd: processing message testmail22177@zensupport.co.uk for ads.norab:1015
May 30 16:25:05 debian spamd[11425]: spamd: clean message (-1.9/5.0) for ads.norab:1015 in 0.5 seconds, 1035 bytes.
May 30 16:25:05 debian spamd[11425]: spamd: result: . -1 - BAYES_00,RCVD_IN_DNSWL_NONE scantime=0.5,size=1035,user=ads.norab,uid=1015,required_score=5.0,rhost=localhost,raddr=127.0.0.1,rport=47805,mid=testmail22177@zensupport.co.uk,bayes=0.000000,autolearn=ham
May 30 16:25:05 debian postfix/local[14522]: F2A8311667: to=ads.norab@debian, orig_to=ads@norab.org, relay=local, delay=1.5, delays=0.03/0.01/0/1.4, dsn=2.0.0, status=sent (delivered to command: /usr/bin/procmail-wrapper -o -a $DOMAIN -d $LOGNAME)
May 30 16:25:05 debian postfix/qmgr[2636]: F2A8311667: removed
source email as bob@alt.zensupport.co.uk
May 30 16:25:05 debian spamd[24828]: prefork: child states: II
May 30 16:25:38 debian postfix/smtpd[14518]: connect from smarthost02.mail.zen.net.uk[212.23.1.2]
May 30 16:25:38 debian postfix/smtpd[14518]: 69E5011667: client=smarthost02.mail.zen.net.uk[212.23.1.2]
May 30 16:25:38 debian postfix/cleanup[14521]: 69E5011667: message-id=testmail33031@zensupport.co.uk
May 30 16:25:38 debian postfix/qmgr[2636]: 69E5011667: from=bob@alt.zensupport.co.uk, size=884, nrcpt=1 (queue active)
May 30 16:25:38 debian postfix/smtpd[14518]: disconnect from smarthost02.mail.zen.net.uk[212.23.1.2]
May 30 16:25:40 debian spamd[11425]: spamd: connection from localhost [127.0.0.1] at port 47807
May 30 16:25:40 debian spamd[11425]: spamd: setuid to ads.norab succeeded
May 30 16:25:40 debian spamd[11425]: spamd: processing message testmail33031@zensupport.co.uk for ads.norab:1015
May 30 16:25:41 debian spamd[11425]: spamd: clean message (-1.9/5.0) for ads.norab:1015 in 0.4 seconds, 1019 bytes.
May 30 16:25:41 debian spamd[11425]: spamd: result: . -1 - BAYES_00,RCVD_IN_DNSWL_NONE scantime=0.4,size=1019,user=ads.norab,uid=1015,required_score=5.0,rhost=localhost,raddr=127.0.0.1,rport=47807,mid=testmail33031@zensupport.co.uk,bayes=0.000000,autolearn=ham
May 30 16:25:41 debian postfix/local[14522]: 69E5011667: to=ads.norab@debian, orig_to=ads@norab.org, relay=local, delay=3, delays=0.03/0/0/2.9, dsn=2.0.0, status=sent (delivered to command: /usr/bin/procmail-wrapper -o -a $DOMAIN -d $LOGNAME)
May 30 16:25:41 debian postfix/qmgr[2636]: 69E5011667: removed
May 30 16:25:41 debian spamd[24828]: prefork: child states: II
May 30 16:25:41 debian postfix/pickup[13686]: D04BF116A0: uid=1015 from=<ads.norab>
May 30 16:25:41 debian postfix/cleanup[14521]: D04BF116A0: message-id=testmail33031@zensupport.co.uk
May 30 16:25:42 debian postfix/qmgr[2636]: D04BF116A0: from=ads.norab@debian, size=1243, nrcpt=1 (queue active)
May 30 16:25:42 debian postfix/smtp[14681]: D04BF116A0: to=baron.ads@gmail.com, relay=gmail-smtp-in.l.google.com[173.194.78.27]:25, delay=1, delays=0.73/0.01/0.04/0.22, dsn=2.0.0, status=sent (250 2.0.0 OK 1369927135 pd10si1250850wjb.181 - gsmtp)
May 30 16:25:42 debian postfix/qmgr[2636]: D04BF116A0: removed
The second (bob@) email has a second phase (May 30 16:25:41 debian spamd[24828]: prefork: child states: II) which never happens when it’s a service@ email…
No idea why - going to guess that there is a generic system wide “service” account that’s causing the problem, but I could be way off the mark!
Cheers,
Adam