Simplify email body

[hishamco]

Fixes #13425

[MikeKry]

@hishamco / @sebastienros

I just came across this change after updating OC. May I ask why did we change this logic? Right now we are unable to send both html version and plain text version in one email - that is imo step in wrong direction. It should be both supported, not exclusively html / plain text.

I would suggest to revert this one, or at least revert changes inside smtp service, so we can create our own email task.. it is good practice to have both html and plain text supported.

[hishamco]

@MikeKry can you clarify how a single email could be send in two different formats?!!

[Mike4tel]

@hishamco well, I believe that it was working all the time before like that :) look up multipart mime type for emails. It is common practice to send both variants of email. 1) it is more likely to pass through spam filters 2) there are still mail clients and users that prefer plain text.

[Mike4tel]

@hishamco for example here: https://help.emarsys.com/hc/en-us/articles/360016272317-Best-practices-Email-size-and-multipart-messages
look for multipart message

[hishamco]

@Mike4tel it depends on the mail software, but while it was supported before I'm going to allow it again

[hishamco]

FYI #14715