Technically, yes. Service becomes a substitute for Software in whatever you
use it for instead of software that you control. You can use a client to
fetch, write and manage your e-mails and contacts, but the messages are still
being stored on someone else's server. Therefore, using your own client does
give you control over some things, but most of the process still goes through
the server.
I personally do use a mail service. I don't consider SaaSS by itself to be as
bad as proprietary software. Taking someone's freedom away is never good and
doing so can and does have severe far-stretched consequences. However, not
everyone can afford or knows how to set up and manage their own mail server.
Then some people offer them services of mail hosting. Some of those are bad
(most of them are), while some of those can be nice, respect the users and
follow noble ideals (Riseup, OpenMailBox?). That still doesn't make them
good, yes, but I wouldn't call them evil. We should try to regain control
over all our services, but these are an acceptable temporary solution for me.
TL;DR: Yes, mail services are SaaSS. If you use a good one, it's acceptable
when better options are not accessible. In my opinion, that is.
