Pretty much a summary of what is happening, Milo, and all covered in post #4 for those who take the trouble to read 'the book'. I have no problems with 'bad' emails from the two ISPs - I think my personal spam filters are adequate there. There are, as you say, issues in the way the hosting outfit is responding to both hack-induced flooding (mostly insecure Wordpress installs) and the odd virus induced spamming and they are (they claim) 'taking steps' to sort it out, BUT as I have said, the major problem is rejection by these two ISPs. I will attempt to find out what barriers my server has in additional to attachment size, exe, com etc. (if, indeed, any, as you are suggesting?). I take it from your comments that some email hosts are actually scanning the entire content of all outbound emails eg Orange? A little concerning in view of recent 'news'.
For those who appear to have either taken the wrong book off the shelf, or lost their place, I repeat part of post #1
I am having infrequent but inconvenient email delivery failures via my current host to Tiscali and AOL addresses. According to 'support', both these ISPs have a 'hair trigger' on spam and it takes very little for them to assess a mail server as a problem.
Is anyone else experiencing problems with these two ISPs?
I have to assume that no-one else is seeing server blocking by these two ISPs. 9/10 times my email server does not appear on any blacklist when the reject from Ti or AOL occurs but it then takes them '24 hours' to clear their block.