My guess is that the tidy extension and/or our xss filter is cleaning 
up the HTML.

Hardly a "High" priority, but if you can provide the actual source of 
the entire email message we can verify/check.

Example is instagram confirm email link - the button is clickable in 
Thunderbird/other email packages but the </a> seems to be moved in 
Horde/IMP

Here is virgin html from email source:

<tr><td style=3D"border-collapse:colla=
pse;border-radius:3px;text-align:center;display:block;border:solid 1px =
#009fdf;padding:10px 16px 14px 16px;margin:0 2px 0 =
auto;min-width:80px;background-color:#47A2EA;"><a href=3D"https://instagra=
m.com/accounts/confirm_email/cHP2C7mk/c3BhcmtwcmVwQGh1bW1pbmdiaXJkLnNvY2lh=
bA/?app_redirect=3DFalse" =
style=3D"color:#3b5998;text-decoration:none;display:block;"><center><font =
size=3D"3"><span style=3D"font-family:Helvetica Neue,Helvetica,Roboto,Aria=
l,sans-serif;white-space:nowrap;font-weight:bold;vertical-align:middle;col=
or:#fdfdfd;font-size:16px;line-height:16px;">Confirm&nbsp;Email&nbsp;Addre=
ss</span></font></center></a></td></tr>

and here is mangled horde version:

<tr>
<td 
style="border-collapse:collapse;border-radius:3px;text-align:center;display:block;border:solid 1px #009fdf;padding:10px 16px 14px 16px;margin:0 2px 0 
auto;min-width:80px;background-color:#47A2EA">
<a 
href="https://instagram.com/accounts/confirm_email/Rbs7zr6B/bnVmYWN0b3JAaHVtbWluZ2JpcmQuc29jaWFs/?app_redirect=False" style="color:#3b5998;text-decoration:none;display:block" target="i_piAjWSr3_N137QkktDtE7" rel="noopener 
noreferrer">
</a>
<center>
<font size="3">
<span style="font-family:Helvetica 
Neue,Helvetica,Roboto,Arial,sans-serif;white-space:nowrap;font-weight:bold;vertical-align:middle;color:#fdfdfd;font-size:16px;line-height:16px">
Confirm&nbsp;Email&nbsp;Address</span></font></center>
</td>
</tr>