Apologies for the inconvenience & a bit of confusion here. Nigel's domain now has correct format of SPF records, you can check the same at :
check SPF
The correct syntax for SPF on Plesk for normal accounts having same email server where they are hosted would be like :
"v=spf1 a mx ptr +all"
OR
<domain>. TXT "v=spf1 a mx ptr +all"
Whereas in case of Nigel, it'd be like :
"v=spf1 ip4:91.186.0.3 a mx ptr +all"
OR
"v=spf1 ip4:%ip% a mx ptr +all"
OR
<domain>. TXT "v=spf1 ip4:<ip> a mx ptr +all"
where
%ip% &
<ip> are the mail server's IP where the emails are working from.