If you use balsa, in Fedora 26 you may have trouble sending mail
(see Bug 1479177 - since upgrade balsa is no longer able to send
balsa uses libesmtp to send the mail, but libesmtp was accidentally
built without SSL support (fixed in my Bug 1483350 - libesmtp built
w/o SSL support -- compat-openssl10 required). balsa itself
should also be built for compat-openssl10.
In the past (e.g., F20 that I upgraded from) a mail server with a
self-signed certificate was accepted, but is now rejected with a
useless error message. I think that a self-signed cert is OK for
submitting email as it is only used to keep the data (and your
password) private (fixed in my Bug 1483649 - balsa no longer accepts
mail server self-signed certificate).
When composing a message, the toolbar Queue button sends immediately
instead of queuing (fixed in my Bug 1483354 - balsa message window
Queue toolbar button sends immediately).