I found this in the forums.
My question is, is there a solution? It is unacceptable to have a custom dictionary keep telling user there is a miss-spelled word after user adds it to custom dictionary. The odd thing is, there are a few (very few) which will be seen as spelled right (which are not in normal dictionary but added to custom).
The link above said there were no plans to fix. This I can’t believe. Clients paying this kind of money and for IBM not to respond to the needs to its customers.
EDIT: The user says custom dictionary worked when we had the old 6.5 mail template. The problem started after updating to 8.5 mail template.