Don't let Safari enter the password for you as you can't see what it's entering (it is possible but we can check that later if necessary). If you get in, you know at least your user name and password are correct. (If you don't, you know one is wrong). In which case, check for the proved correct user name in Mail Preferences and enter the proved correct password instead of what's there. If that doesn't work, post again.
I tried to find the correct server settings online and found two sites offering conflicting information so go by what works on your iOS devices.
Password checked via the link given Mick and it worked. Then I entered the proved password in Mail Prefs. I sent myself emails from the iPad but the toucansurf emails don't come in on the iMac. Btw, the format for settings on the iMac makes it difficult to do a full comparison with settings on the iPad and iPhone. Perhaps I am not on the correct page.
Have checked the Accounts section again on the iMac and iPad. The Incoming Mail Server settings are the same. Only one aspect is different: the iMac is a POP Account but the iPad is headed as an IMAP Account. However the Host Name on both is pop3.toucansurf.com.
pop3… suggests a pop account. Do you have a phone? If so, send an email from the iPad. If it appears in the sent box on BOTH, then it's an IMAP account. If it's only in sent on the iPad, it's pop.
What server ports are being used? That may also give us a clue. I assume they're the same on both devices as you say the settings are the same but can you double check? Can you find out online what they should be?
Email sent from iPad to iPhone doesn’t appear in the sent box of the phone, so it’s a POP account.
The email isn’t received on the iMac (which is the problem) and it isn’t in the sent box on the iMac.
Server port on iPhone is 110 for toucansurf account. Can’t find the port on iPad or iMac.
One aspect noted is that the iMac is not listed in ‘MY NETWORKS’ under wi-fi in settings on the iPad. (the iPhone is listed).