In my (limited) experience using i2pmail.org as a clearnet relay, I find that an expected email may not hit my @mail.i2p inbox until some hours later. I'm not certain if this delay is caused from the clearnet to I2P path or within the I2P network itself.

Still a very suitable and beneficial service, of course. But something to consider for time sensitive emails, as using that email address for something you may need a password reset on or email based auth-code that expires after XX minutes, you may find yourself in between a rock and a hard place.

EDIT: Disregard the above about it not being suitable for time-senstive emails. I just did a test, and received my clearnet email to my I2P inbox in about 5 minutes. In the past, it had taken hours, but in hindsight, the webmail inbox may have been cached or it could have just been a one-off fluke. ¯\_ (ツ)_/¯?