[andrei_says_]

Ironically <span style="font-size: 0px;"> is a must for spacing tables if you want outlook compatibility.

[citrin_ru]

Ironically this technique was used in phishing for MS Office 365 credentials (users of which often use MS Outlook): https://www.avanan.com/blog/zerofont-phishing-attack

Rendering in Outlooks changes from time to time - it is worth to check if this workaround is still required.