Code in Evolution is available under a variety of licenses. Each file should have a license header stating the license. In the absence of that code is available under the LGPLv2 or (at your choice) LGPLv3. See COPYING.LGPL2 and COPYING.LGPL3 for the full license texts.
There are a few exceptions to this rule that have an impact on overall licensing. At the present, the code seems GPLv2+ clean.
OpenLDAP license
addressbook/gui/component/openldap-extract.h
GPLv2+
widgets/misc/e-spinner.c
plugins/calendar-http/calendar-http.c
plugins/calendar-weather/calendar-weather.c
LGPLv2 only
plugins/webdav-account-setup/webdav-contacts-source.c
LGPLv2+
widgets/misc/e-icon-entry.[ch]
widgets/text/e-text.[ch]
widgets/table/e-cell-tree.[ch]
widgets/misc/e-dateedit.[ch]
MPL/GPL/LGPLv2+
smime/lib/e-asn1-object.c
smime/lib/e-cert.c
smime/lib/e-cert-db.c
smime/lib/e-cert-trust.c
smime/lib/e-pkcs12.c
In addition there are a few pieces that lurk in the tree, but are not linked as part of evolution, eg. test code:
GPLv2 only
addressbook/gui/widgets/test-reflow.c
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