When using the automation action Send customized email, a hidden tracker image is added to the end of the email notification.

Content added similar to the below:

<img alt="" src="http://atlassian-trk.prd.msg.ss-inf.net/CI0/010101983dd4b8d0-8edf738d-8661-4dc0-9d84-abadf0cba048-000000/FYvUZpID__PM2xsJN704XFq9zzSezz3Kgl8--mUdBcc=415" style="display: none; width: 1px; height: 1px;">

It can cause some issues for email clients when opening the notifications:

Our suggestion is to remove this image.

Workaround

Use the "send email" action instead of "send customized email."