to answer that question is a bit like trying the answer the proverbial "How long is a piece of string?" There are literally many many variables and the server sending and the server being sent to all can have their own policy reasons for rejecting mail. So as a starting point what mail provider is sending the mail and the error message will name a server that is rejecting the mail, so please post the error message verbatim so we will know what it says in it's entirety.

Thanks for your very prompt reply. As it happens I tried re-sending the saved message this morning and it went without a problem. The previous time that this occurred the message was blocked every time I tried, even after majorly changing the content. FYI I am using Thunderbird, and the error message just referred to 'the server'. For now, the problem is solved.