emails with attachments not received by recipient when sent to either a me.com or iCloud.com email address

I have an issue that seems to have started over the last week.

If I send an email to an apple email address (xxx@me.com or xxx@icloud.com) from either my work or me.com account, and the email has either a signature and/or an attachment, it is not received by the recipient (yet it does show in my sent items and I do not get a bounce back). As soon as I delete the attachment and/or the signature, it goes straight through and is received by the recipient.

I have tried sending this to 2 x apple email accounts (1 x @me.com and 1 x @icloud.com) and both have the same issue. I've also tried sending emails to my own @me.com account (from both my work email address and my @me.com email) with no luck (although the emails with no attachments come straight through)

I use Outlook for Mac 2011 but have also tried this with the iCloud web app (both to send and receive) so I'm assuming this is an iCloud issue as opposed to Microsoft?

Can anyone help?

Many thanks