[Android Apps and Games] Chris.B.:
Indeed Gandi support wildcard aliases and I created several like ab*[at]mydomain.fr

Here is the source(anonymised) of an email sent with Thunderbird through an identity abtest[at]mydomain.fr.

Like you will see, my real address only appears in "authenticated sender" whereas my alias is everywhere else.

Delivered-To: junkmail@gmail.com
Received: by xxxx:xxx:xxx:0:0:0:0:0 with SMTP id y23csp950xxxxxx;
Fri, 1 Nov 2019 14:48:56 -0700 (PDT)
X-Google-Smtp-Source: xxxxxqzlxV42ByI1CTc+YtJOj6hFD/UHILMgmmi4Xwl0Rzt1hAPGxdZJRpY98EdKkaxxxxxxxxxx
X-Received: by 2002:a17:906:229a:: with SMTP id xxxxx12020375eja.76.157xxxxxxxxxx;
Fri, 01 Nov 2019 14:48:56 -0700 (PDT)
ARC-Seal: i=1; a=rsa-sha256; t=15726xxxxx; cv=none;
d=google.com; s=arc-20160816;
b=NwXcsG/LKruQnI2HNh36WyFHo/6OdOvFfkyROR/hE57sno9aAHvnaNhdRK3J6rz12v
DU1Lo5pxXvGeHTo90BHGDz65mjxJkCxbNWQ2W/H34Mo1+105seQh7tYit8H9RsgHwSLX
MHL45wnP8m1q3huEarMD++OUAOdYtgPgLmKEiFnx9PONYEUW2zQ2U9YCTNxfYbogGvYQ
U04+ah6x0BWRpFaYyqzmZ6gEXUjzowSIH4KZYUBOzjGrx5iWveAq1MZm+INZ+omX5eNx
wlKXfmXdVqrX7PAr9LbTll0jqTtaxeeTpHqUewqZqXO0lBx7gshif5P6zvveJ+X4hJax
xxxxxx
ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=arc-20160816;
h=content-language:mime-version:user-agent:date:message-id:subject
:from:to;
bh=8Z3ig5o/Jlib2OIGnLPRra93i9c7YjSiE5xYQJXgvRk=;
b=eAktgOH2YzJT5hqJmUB7jEC8ZNbjJi1iIoIjuQ8ZKsJ6vKYJKlKRvNa/hatw6pF1IM
7PWLmiC4GZLtOUQS4jQ+a4BZzmBP2Pq29hqgikT4fP4lgsixZDkyLif4sOA4RekrLcVJ
bswV6cS6HAfJBLcM4Kwu+3F+astai2g7T9LmiNG1sw7ixjHCW7Ny6Nvo1xyyC805Ip5Z
H7zUM1RJSTnbYfU/Suw7uaMleH5gSJxedq6Pod2DugrpfMzQqezJczqks+0BKwHyOLGX
joxpVkzRGHsCEbMtFAxFZ2Y5sbLRdoDLMUzZP2xmr8kPCQ3PAIK8Yg/8dOHbi0AKphCT
xxxxxx
ARC-Authentication-Results: i=1; mx.google.com;
spf=pass (google.com: domain of abtest@mydomain.fr designates xxx.xxx.xxx.xxx as permitted sender) smtp.mailfrom=abtest@mydomain.fr
Return-Path: <abtest@mydomain.fr>
Received: from relay9-d.mail.gandi.net (relay9-d.mail.gandi.net. [xxx.xxx.xxx.xxx])
by mx.google.com with ESMTPS id xxxxxx965163eje.380.2019.11.01.xx.xx.xx
for <junkmail@gmail.com>
(version=TLS1_2 cipher=ECDHE-RSA-CHACHA20-POLY1305 bits=256/256);
Fri, 01 Nov 2019 xx:xx:xx -0700 (PDT)
Received-SPF: pass (google.com: domain of abtest@mydomain.fr designates xxx.xxx.xxx.xxx as permitted sender) client-ip=xxx.xxx.xxx.xxx;
Authentication-Results: mx.google.com;
spf=pass (google.com: domain of abtest@mydomain.fr designates xxx.xxx.xxx.xxx as permitted sender) smtp.mailfrom=abtest@mydomain.fr
X-Originating-IP: xxx.xxx.xxx.xxx
Received: from [xxx.xxx.xxx.xxx] (unknown [xxx.xxx.xxx.xxx])
(Authenticated sender: firstnamelastname@mydomain.fr)
by relay9-d.mail.gandi.net (Postfix) with ESMTPSA id E8919xxxxx
for <junkmail@gmail.com>; Fri, 1 Nov 2019 21:48:55 +0000 (UTC)
To: junkmail@gmail.com
From: firstname lastname <abtest@mydomain.fr>
Subject: test sent mail
Message-ID: <334axxxx-1011-7fxx-77xx-470dfcxxxxxx@mydomain.fr>
Date: Fri, 1 Nov 2019 22:48:55 +0100
User-Agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:68.0) Gecko/20100101
Thunderbird/68.2.0
MIME-Version: 1.0
Content-Type: multipart/alternative;
boundary="------------25CBD183AD81BB6989xxxxxx"
Content-Language: fr

This is a multi-part message in MIME format.
--------------25CBD183AD81BB6989xxxxxx
Content-Type: text/plain; charset=utf-8; format=flowed
Content-Transfer-Encoding: 7bit

test sent mail with identity

--------------25CBD183AD81BB6989xxxxxx
Content-Type: text/html; charset=utf-8
Content-Transfer-Encoding: 7bit

<html>
<head>

<meta http-equiv="content-type" content="text/html; charset=UTF-8">
</head>
<body>
<font face="Calibri">test sent mail with identity<br>
</font>
</body>
</html>

--------------25CBD183AD81BB6989xxxxxx--

I already use several aliases daily. I used to create more than 10 identities in my old app.

With yours, I change the sender address each time.

It's easier but the best will be that the sender address is directly the alias.

It works well, my recipients answer me on my alias and they don't see my real address (who really read email source :silly:).

If you see nothing abnormal, I will continue by editing the sender address because your app remains the easiest solution for me, in addition to all its other advantages !

Edit : Maybe the beginning of an explanation.

Here is the source (anonymised) of an email sent to abtest02[at]mydomain.fr

Return-Path: <junkmail@gmail.com>
Delivered-To: firstnamelastname@mydomain.fr
Received: from spool.mail.gandi.net (spool5.mail.gandi.net [xxx.xxx.xxx.xxx])
by nmboxes122.sd4.0x35.net (Postfix) with ESMTP id DAA5Exxxxx
for <firstnamelastname@mydomain.fr>; Fri, 1 Nov 2019 17:51:37 +0000 (UTC)
Received: from mail-wr1-f53.google.com (mail-wr1-f53.google.com [xxx.xxx.xxx.xxx])
by spool.mail.gandi.net (Postfix) with ESMTPS id BE193xxxxxx
for <abtest02@mydomain.fr>; Fri, 1 Nov 2019 17:51:37 +0000 (UTC)
Received: by mail-wr1-f53.google.com with SMTP id xxxxxx0392211wra.6
for <abtest02@mydomain.fr>; Fri, 01 Nov 2019 10:51:37 -0700 (PDT)
DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed;
d=gmail.com; s=20161025;
h=to:from:subject:message-id:date:user-agent:mime-version
:content-language;
bh=xxxxxxxxxxsuGD8/VMgeng0KF+pNzPtYf9kAD1Dv3sA=;
b=o9SS20FUgO4+5nozyv2eh0wZNCElF+hUtl3q7FF6riwX7PW5C5XSvZ0mraOUMOfEbO
07/thU8bUVL/94vzUKYfpJwC52AswwEWSLt8OoI7n+mirBHy1VGf2ihzAx9uEl5AAydY
zpMlXL/RNhcbjaF6Q7s9yVl96k8tgL2is9jE1lh5P18hMGorwG3gh07nmlmGVdVtYCeA
eMk+D98wQxwyGvUxHgTLD0dC07PX48PeRW6fBrfK/cWl+ZLHdDLfRvK+5HnCPMz1PcRL
/aa6be7ZzEibSYk1X7DWGBm52tMSFftqB17MKSLNQ4cRINMZ5XJSsQwV4uZL3SL7tr9a
xxxxxx
X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed;
d=1e100.net; s=20161025;
h=x-gm-message-state:to:from:subject:message-id:date:user-agent
:mime-version:content-language;
bh=xxxxxxxxxxsuGD8/VMgeng0KF+pNzPtYf9kAD1Dv3sA=;
b=TOSi83hRzhbwXnjiurES5imjOw3p6JKIQOy5wuFcZEG9bSrjCW3soIpLdELawSRZw+
zkXvzx0yEMffO+JaW0gu+B4Z4fBrAEQAZqP6s9Ki7PbECSLNMcOd+Y05zvLuJYAyKEl4
Xf08CfFr3Zd1JzMuJzkt7YAaoZfmFlNZpxwbdo6XaJoiE9aSYZPIBS6xdJG6woYBTihK
0iQAYKiO903qhKcwcO6fxXMPvK3F7w8uCroJLio3xNOY1ws6udIQbiWc2gMIhJ55Mdqh
GnAWtOAinkLfEd4v4bTq1V7GWMHWQ7mpcXXmxS5+JedPzzpDAAtspZENyxlDZA8jfNe7
xxxxxx
X-Gm-Message-State: xxxxxxUdVxrpHqoLRunEchlGawLV0856fRwH9npltYpjQGkCQwumUJ8b
QHwh+rOZLOzijGB2P4xxxxxxxxxx
X-Google-Smtp-Source: xxxxxxxxxx67SpfbZYvFcyMyNFpB5N6QDVBv6cdIywQiHz1WJ89Xcgs++lBuU16xNHZKoUxxxxxxxxxx
X-Received: by 2002:a5d:4885:: with SMTP id xxxxxx879642wrq.219.1572630xxxxxx;
Fri, 01 Nov 2019 10:51:36 -0700 (PDT)
Received: from [xxx.xxx.xxx.xxx] ([xxx.xxx.xxx.xxx])
by smtp.googlemail.com with ESMTPSA id xxxxxx13014289wmb.1.2019.11.01.10.51.35
for <abtest02@mydomain.fr>
(version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128);
Fri, 01 Nov 2019 10:51:36 -0700 (PDT)
To: abtest02@mydomain.fr
From: junkmail <junkmail@gmail.com>
Subject: test02
Message-ID: <48exxxxx-eaxx-49xx-bfxx-dbeba8xxxxxx@gmail.com>
Date: Fri, 1 Nov 2019 18:51:35 +0100
User-Agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:68.0) Gecko/20100101
Thunderbird/68.2.0
MIME-Version: 1.0
Content-Type: multipart/alternative;
boundary="------------4183905F9FEF9B0BEExxxxxx"
Content-Language: fr
X-GND-Status: LEGIT
Received-SPF: pass (spool5: domain of gmail.com designates xxx.xxx.xxx.xxx as permitted sender) client-ip=xxx.xxx.xxx.xxx; envelope-from=junkmail@gmail.com; helo=mail-wr1-f53.google.com;

This is a multi-part message in MIME format.
--------------4183905F9FEF9B0BEExxxxxx
Content-Type: text/plain; charset=utf-8; format=flowed
Content-Transfer-Encoding: 7bit

test02

--------------4183905F9FEF9B0BEExxxxxx
Content-Type: text/html; charset=utf-8
Content-Transfer-Encoding: 7bit

<html>
<head>

<meta http-equiv="content-type" content="text/html; charset=UTF-8">
</head>
<body>
<font face="Calibri">test02</font>
</body>
</html>

--------------4183905F9FEF9B0BEExxxxxx--

Like you can see, the field "Delivered-To:" is filled with firstnamelastname[at]mydomain.fr whereas the email is sent to abtest02[at]mydomain.fr (which is in the "To:" field).

Could it explain the behaviour of the app which fills the sender address with the email in the identity(or matching with the regex) because it is based on the "Delivered-To:" field ?