iCloud mail with attachments not getting delivered

[zany.ninja]

Have had this problem for a few days. Any email sent from my iCloud mail account with attachments (teeny tiny PDF and RTF files) is not getting delivered - have tried sending to my Outlook and Gmail account, but nothing. I used my iMac with Sierra, MacBook Air with High Sierra, iPad 2 with iOS 9 - same thing happened on all of them. Any ideas what's going on??

 

[zany.ninja]

Turns out it was something to do with one of the PDF files I was trying to attach. Any email with that particular file attached (just 32KB) either wasn't being sent by the iCloud mail server, or received by the recipient, for whatever reason.

Attaching and sending the same PDF file in Gmail was fine though.

Weird.