Please help me to choose a domain name!

I would definitely type email as a keyword to find that kind of service, so if SEO is a matter for you, go for that one :)

You can try searching for the number of Google search results for both the keywords and go with the one that provides the least number of results. It will probably be easier to rank for the one with the least number of search results.

I hope you've already purchased those domains. I'm amazed at how many domain swatters there are out there to make a quick buck. All the best!

Is email your only medium? Is communication only inbound?

I like the latter one :)

inboxniche.com - I'm assuming all the social accounts are available for both.

Catch: the actual word is niche itself. Who is the first to rush hover.com? :P

email, though it does depend a lot on what that second word actually is

inbox

inboxniche. Sounds interesting... how will you get people to opt in and give their emails? I'd be interested in using this service for my business.

emailniche.com

email