I tried copying the email content then pasting it into a new email to the same recipients but with a slight difference in the subject (I don't know if that actually did anything useful or not) and it then sent without any problem. It's not the first ...

One way around this problem is to copy the contents of the email and send it by Smash, which is a free service for sending larger files than email can handle, but can be used for any emails regardless of size. It's a very reliable and safe service th...