Thanks very much, Black Mamba, for your assistance. It is much appreciated.
But unfortunately none of the suggestions listed - although all plausible - are applicable in this case. For example, there is enough storage, the attachment is about 1.6 MB, and the attachment was actually sent. In fact, the attachment appears in a screenshot of my return message to the sender, although in my mailbox I cannot see it!
I have spoken to someone technical on the Bluewin Webmail Hotline who also could not solve the problem. The technical support person deleted all my browser cookies, cache, etc. and tried every which way, but in the end got back to square one. He then escalated it to a higher level within their technical department. However, I have yet to hear back from them.
I will try and access the attachment by asking someone else to forward it to me and see if it works. Strangely enough, someone else who uses Bluewin received the attachment, but maybe she is a full member of Swisscom, whereas I now only have my phone number and email with Swisscom and Bluewin, whereas my children's mobile phones as well as our family's TV and internet are with a different service provider.
What worries me most is that I may not even realize that there ought to be an attachment, because I cannot see it. If I do not look at the size of the email, nothing alerts me to the fact that there should actually be an attachment.
However, enough written about my issue. What I want to say is that I am thankful for your help with my problem.
Best wishes