Accounts: do not auto-sync for certain custom server based accounts

Accounts based on non /e/ custom servers are not to be synced because they
most probably lack a mail server in their implementation. So check
whether an /e/ type tagged account is syncable or not and let the mail
app to not crash if it isn't tagged syncable.

Signed-off-by: Sumit Pundir <pundirsumit11@gmail.com>