From en.wiktionary.org:
[mail]
** German
*** Etymology
Abbreviation of [de]; the _E-_ being redundant in German.
*** Pronunciation
- [de]
- [de] [according to the latter pronunciation]
- [de]
*** Noun
[f[ predominantly ] ,s] _or_<br /> [n[ alternatively in southern Germany,
Austria and Switzerland ] ,s,s]
1. [de] [de]
**** Usage notes
- Usually feminine; see [de] for more.
- The form [de] is predominant in informal contexts, but is now also a common variant in formal language. In the prestigious newspaper [Die Zeit] , the short form was used about 28% of the time between 2009 and 2018, while the full form was used 72% of the time. [1] [2]
**** Declension
[f[ predominantly ] ,s] _or_<br /> [n[ alternatively in southern Germany,
Austria and Switzerland ] ,s,s]
*** References
References:
[1]. Korpustreffer für "@Mails", aus dem Korpus Die ZEIT des Digitalen Wörterbuchs der deutschen Sprache (see https://www.dwds.de/r?q=%40Mails&corpus=zeit&date-start=2009&date-end=2018&format=full&sort=date_desc&limit=50)
[2]. Korpustreffer für "@E-Mails", aus dem Korpus Die ZEIT des Digitalen Wörterbuchs der deutschen Sprache (see https://www.dwds.de/r?q=%40E-Mails&corpus=zeit&date-start=2009&date-end=2018&format=full&sort=date_desc&limit=50) < !-- Using the plural form to avoid names like "Daily Mail" etc. -->
[de]