Problem sending emails

You'll need to see what the bounceback message says.
Usually it's because of one of the following:

If this is an organisational email address you are sending from/to it may be worth contacting the IT administrator for that organisation to check.

Thanks that is helpful.

Most of those I can rule out. If there is a connectivity error does that mean I can't send any email from my account? I have been able to send so not sure why this one is causing issues.

Can the receipient block emails for vairous reasons?

Usually if it's a connection issue you wouldn't be able to access the internet generally (albeit it may have been temporary).
I did forget to mention one thing I've also seen is sometimes certain mail servers are set to not just file emails identified as spam in their spam folder, but to prevent it actually being received and send an automatic reply saying it got filtered.
Not sure about blocking thing. It probably is possible although I expect more typical probably from organisations rather than individuals (where they might just black list certain email providers they know tend to be sources of spam and/or malware).

I have sent emails in the past to this address and had replies back, so it shouldn't be spam right?