To set a nice HTML signature in Mail.app, you have to find the location of your signature in the Library folder.
For me, in Yosemite, the path was ~/Library/Mobile\ Documents/com\~apple\~mail/Data/MailData/Signatures
.
The path could also be ~/Library/Mail/V7/MailData/Signatures
(for Catalina)
The signature is located in a file that has the following pattern: ubiquitious_*.mailsignature
The HTML signature can be found below. Adopt to your liking. The file must be locked or Mail will overwrite.
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<meta name="format-detection" content="telephone=no">
<p style="font-family: Arial, sans-serif; font-size: 10pt; line-height: 16px; color: rgb(33, 33, 33);">
<span style="font-weight: bold; color: rgb(61, 59, 59); display: inline;">Dr Christophe Trefois, Dipl.-Ing.</span>
<span style="display: inline;"></span><br/>
<span style="color: rgb(61, 59, 59); display: inline; font-size:7.5pt;">Technical Specialist / Post-Doc</span>
</p>
<p style="font-family: Arial, sans-serif; font-size: 7.5pt; line-height: 16px;">
<span style="font-weight: bold; color: rgb(61, 59, 59); display: inline;">UNIVERSITÉ DU LUXEMBOURG</span><br/>
<span style="display: inline;"><br></span>
<span style="font-weight: bold; color: rgb(61, 59, 59); display: inline;">LUXEMBOURG CENTRE FOR SYSTEMS BIOMEIDINCE</span><br/>
<span style="color: rgb(61, 59, 59); display: inline;">Campus Belval | House of Biomedicine <br/>
7, avenue des Hauts-Fourneaux <br />
L-4362 Esch-sur-Alzette
</span> <br/>
<span style="color: rgb(61, 59, 59); display: inline;">T: </span> <span style="color: rgb(61, 59, 59); display: inline;">+352 46 66 44 6124</span> <br/>
<span style="color: rgb(61, 59, 59); display: inline;">F: </span> <span style="color: rgb(61, 59, 59); display: inline;">+352 46 66 44 6949</span>
<span style="display: inline;"><br></span>
<a href="http://www.uni.lu/lcsb" style="color: rgb(0, 109, 189); display: inline;">http://www.uni.lu/lcsb</a>
</p>
<p style="font-family: Arial, sans-serif; font-size: 14px; line-height: 16px;"><a href="https://www.facebook.com/trefex" style="display: inline;"><img width="24" height="24" data-filename="facebook.png" src="https://s3.amazonaws.com/htmlsig-assets/rounded/facebook.png" alt="Facebook"></a>
<a href="https://twitter.com/Trefex" style="display: inline;"><img width="24" height="24" data-filename="twitter.png" src="https://s3.amazonaws.com/htmlsig-assets/rounded/twitter.png" alt="Twitter"></a>
<a href="https://plus.google.com/+ChristopheTrefois/" style="display: inline;"><img width="24" height="24" data-filename="googleplus.png" src="https://s3.amazonaws.com/htmlsig-assets/rounded/googleplus.png" alt="Google Plus"></a>
<a href="https://www.linkedin.com/in/trefoischristophe" style="display: inline;"><img width="24" height="24" data-filename="linkedin.png" src="https://s3.amazonaws.com/htmlsig-assets/rounded/linkedin.png" alt="Linkedin"></a>
<a href="http://skype:Trefex?call" style="display: inline;"><img width="24" height="24" data-filename="skype.png" src="https://s3.amazonaws.com/htmlsig-assets/rounded/skype.png" alt="skype"></a>
</p>
<p style="font-family: Arial, sans-serif; color: rgb(61, 59, 59); font-size: 9px; line-height: 16px;">
----<br/>
This message is confidential and may contain privileged information. <br/>
It is intended for the named recipient only. <br/>
If you receive it in error please notify me and permanently delete the original message and any copies. <br/>
----<br/>
</p>
And this is how it looks in the email client:
Dr Christophe Trefois, Dipl.-Ing.
Technical Specialist / Post-Doc
UNIVERSITÉ DU LUXEMBOURG
LUXEMBOURG CENTRE FOR SYSTEMS BIOMEIDINCE
Campus Belval | House of Biomedicine
7, avenue des Hauts-Fourneaux
L-4362 Esch-sur-Alzette
T: +352 46 66 44 6124
F: +352 46 66 44 6949
----
This message is confidential and may contain privileged information.
It is intended for the named recipient only.
If you receive it in error please notify me and permanently delete the original message and any copies.
----
You can use SVG instead of img. It will display normally in almost all mail clients: http://sharingbuttons.io. And your mail will load faster ;)