If the intended recipients are using email clients (such as Outlook, Outlook Express, Windows Live Mail or Thunderbird) ask them to access their email accounts via their email providers' websites (rather than through those clients).
They should then look for a folder called 'Bulk', 'Junk' or something similar. There's a very good chance that your emails will be sitting in there. (i.e. their email providers' anti-spam filters have erroneously identified your mail as spam, and prevented them from being forwarded to the intended recipients).
[If they normally use webmail anyway, rather than email clients, they should still look for 'Bulk', 'Junk' or other folders; they can be easily ignored when viewing an Inbox].
Such problems are extremely common when the sender and intended recipient are in different countries. (For example, I repeatedly had difficulties emailing another AB member in the USA. If I found an email provider whose services weren't blocked at his end, that provider then blocked his return mails at my end). While the problem occurs less frequently when sending from one UK-based email address to another one, it does still occur.
Chris