Questions & Answers about Jag vet inte vad mejlet gäller.
Here, gäller means something like concerns or is about.
- gälla = to concern, apply to, be about
- gäller = present tense
So the sentence uses gälla in the sense of what the email concerns / what the email is about.
In everyday Swedish, many people would also say handlar om:
- Jag vet inte vad mejlet handlar om.
That version is often a bit more conversational.
Gäller is the present tense of the verb gälla.
Some useful forms are:
- infinitive: gälla
- present: gäller
- past: gällde
- supine: gällt
So in this sentence, the speaker is saying what they do not know now.