You a right, finally, a simple admission that TPG is simply not as great as the other larger email providers and cannot do what they do would have sufficed rather than the insistence that TPG does just fine as a stand alone email provider.... I will transfer my TPG email to another than can do what I want it to do... thanks anyway.
I really just only registered here to inform everyone that I had the same problem (different provider - this came up on google as I was searching for the solution).
After hours of trying to find the problem on my IMAP/SMTP server I realized that Spark is a piece of ** app and has a messed up DNS resolver. Our server's DNS name did not work when you input it in the app, but if you put it's IP address everything works without issues. Spark eat ** honestly.
I do apologize for my bad language, but I was upset for all the wrong reasons. Also OP is an **.
I have just SOLVED this exact problem which was driving me insane for about 2 hours so I thought I'd share...
Turns out everything I was inputting in the fields was correct. However, there appeared to be an invisible space after pasting the email server (i.e: smtp.ionos.co.uk) into the relevant field. When removed it worked fine. Maybe it is a similar issue for you guys.