exceed a total of 1GB (Gigabytes) of email on our servers

[#1]

Hi, I have had an email stating the above I have a 2 main emails (IMAP) and I use outlook on my PC and pick them up on my phone also. When I go into webmail sent items I have a email which a 5mb attachment which has 'copied' itself hundreds of time and therefore taking up this space. I have deleted it on webmail but it reappears this item is not showing on my phone or outlook. Any ideas of how to delete it?
Many Thanks

[#2]

have you emptied the deleted items in webmail?

[#3]

They email in question is not there just 398 pages of the same message in sent items - delete them in sent not showing in trash / some time later they are back in sent?

[#4]

Are they only reappearing after you've started outlook on your PC or looked at the mail on your phone?