Firstly, it doesn’t have to be all at once, do it in stages.
Also, when I switched I used it as an opportunity to organize my emails, so I made a finances tag, a medical tag, etc. I would make a tag and move over all the emails that fit that tag, eventually I had moved everything over. It also ended up being a good chance to close accounts I hadn’t used before.
Probably your lemmy instance censoring a word contained within what was written.