Letter Font while sending mass emails

Letter Font while sending mass emails

Hello everyone, I have been sending different mass emails through Bigin the last days and I noticed that the font is changing sometimes. Even if I see that the email is well written and at the desired font and size, the recipients are receiving it in different format. Do you know what is happening?
I have already set up the composer settings with specific font and size, but sometimes it doesn't work.
Attached part of the email.