Not a DOI
The resolution service at https://doi.org resolves DOIs. You have attempted to resolve mailto:michail.rassias@math.uzh.ch, which is not a DOI.
mailto:michail.rassias@math.uzh.ch may be a Handle (DOIs are a special sort of Handle). If mailto:michail.rassias@math.uzh.ch is a valid Handle you should be able to resolve it at https://hdl.handle.net.
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