Approvers Limit?

Hi,

Where are you getting the approvers from, SharePoint group, multi select person field in an approvers list or are you manually entering them at some point?

SharePoint has a limit of 10Bytes on a person field, but from what I've read this is only reached around 200 entries. 

If you are storing them in a string, then there is a limit of 255 characters

Try storing them in a multiline variable which has a much larger limit

thar might be limited on exchange side, but it's defaulted to much higher number. 

https://docs.microsoft.com/en-us/exchange/mail-flow/message-size-limits?view=exchserver-2019

but from the error message I'd rather suspect whether you do not have a deleted or disabled account on the list, which mail address cannot be resolved for.

or whether your query doesn't return an empty element for any reason.

it returns me something

check the exact content.

as mentioned, some DB records may miss mail address which then may turn into empty collection element, or mail address need not ne in correct format (eg. an extra space), or there may be stored multiple addresses where there is expected just one.

make sure query returns addresses without a separator (comma, semicolon...) - nintex (sometimes) interprets them as a part of the address

If I have some doubloons in my mail list, can it cause any trouble?

duplicates (duplicate mail addresses) shouldn't cause any problem.

if you have the list in collection you can try to 'Remove duplicates' operation